{"id":887,"date":"2025-08-13T19:24:50","date_gmt":"2025-08-13T13:54:50","guid":{"rendered":"https:\/\/codeanddebug.in\/blog\/?p=887"},"modified":"2025-08-13T19:24:51","modified_gmt":"2025-08-13T13:54:51","slug":"perfect-sum-problem","status":"publish","type":"post","link":"https:\/\/codeanddebug.in\/blog\/perfect-sum-problem\/","title":{"rendered":"Perfect Sum Problem | All 4 DP Solutions in Python"},"content":{"rendered":"\n<p>Given an array&nbsp;<strong><code>arr<\/code><\/strong>&nbsp;of non-negative integers and an integer&nbsp;<strong><code>target<\/code><\/strong>, the task is to count all subsets of the array whose sum is equal to the given&nbsp;<code>target<\/code>.<\/p>\n\n\n\n<p>Here&#8217;s the [<strong><a href=\"https:\/\/www.geeksforgeeks.org\/problems\/perfect-sum-problem5633\/1\" target=\"_blank\" rel=\"noreferrer noopener\"><mark style=\"background-color:rgba(0, 0, 0, 0)\" class=\"has-inline-color has-vivid-purple-color\"><span style=\"text-decoration: underline;\">Problem Link<\/span><\/mark><\/a><\/strong>] to begin with.<\/p>\n\n\n\n<p><strong>Examples:<\/strong><\/p>\n\n\n\n<pre class=\"wp-block-preformatted\"><strong>Input<\/strong>: arr[] = [5, 2, 3, 10, 6, 8], target = 10\n<strong>Output: <\/strong>3\n<strong>Explanation<\/strong>: The subsets {5, 2, 3}, {2, 8}, and {10} sum up to the target 10.<\/pre>\n\n\n\n<pre class=\"wp-block-preformatted\"><strong>Input<\/strong>: arr[] = [2, 5, 1, 4, 3], target = 10\n<strong>Output:<\/strong> 3\n<strong>Explanation<\/strong>: The subsets {2, 1, 4, 3}, {5, 1, 4}, and {2, 5, 3} sum up to the target 10.<\/pre>\n\n\n\n<pre class=\"wp-block-preformatted\"><strong>Input<\/strong>: arr[] = [5, 7, 8], target = 3<br><strong>Output:<\/strong> 0\n<strong>Explanation<\/strong>: There are no subsets of the array that sum up to the target 3.<\/pre>\n\n\n\n<pre class=\"wp-block-preformatted\"><strong>Input<\/strong>: arr[] = [35, 2, 8, 22], target = 0<br><strong>Output:<\/strong> 1\n<strong>Explanation<\/strong>: The empty subset is the only subset with a sum of 0.<\/pre>\n\n\n\n<p><strong>Constraints:<\/strong><br>1 \u2264 arr.size() \u2264 10<sup>3<br><\/sup>0 \u2264 arr[i] \u2264 10<sup>3<\/sup><br>0 \u2264 target \u2264 10<sup>3<\/sup><\/p>\n\n\n<div style=\"max-width: -moz-fit-content; \" class=\"wp-block-ub-table-of-contents-block ub_table-of-contents ub_table-of-contents-collapsed\" id=\"ub_table-of-contents-2eedd990-b061-4681-9d5b-b509fc889568\" data-linktodivider=\"false\" data-showtext=\"show\" data-hidetext=\"hide\" data-scrolltype=\"auto\" data-enablesmoothscroll=\"true\" data-initiallyhideonmobile=\"true\" data-initiallyshow=\"false\"><div class=\"ub_table-of-contents-header-container\" style=\"\">\n\t\t\t<div class=\"ub_table-of-contents-header\" style=\"text-align: left; \">\n\t\t\t\t<div class=\"ub_table-of-contents-title\">Contents:<\/div>\n\t\t\t\t<div class=\"ub_table-of-contents-header-toggle\">\n\t\t\t<div class=\"ub_table-of-contents-toggle\" style=\"\">\n\t\t\t\u00a0[<a class=\"ub_table-of-contents-toggle-link\" href=\"#\" style=\"\">show<\/a>]\n\t\t\t<\/div>\n\t\t<\/div>\n\t\t\t<\/div>\n\t\t<\/div><div class=\"ub_table-of-contents-extra-container\" style=\"\">\n\t\t\t<div class=\"ub_table-of-contents-container ub_table-of-contents-1-column ub-hide\">\n\t\t\t\t<ul style=\"\"><li style=\"\"><a href=\"https:\/\/codeanddebug.in\/blog\/perfect-sum-problem\/#0-solution-1-recursion-brute-force\" style=\"\">Solution 1: Recursion (Brute Force)<\/a><ul><li style=\"\"><a href=\"https:\/\/codeanddebug.in\/blog\/perfect-sum-problem\/#1-intuition-and-approach\" style=\"\">Intuition and Approach<\/a><\/li><li style=\"\"><a href=\"https:\/\/codeanddebug.in\/blog\/perfect-sum-problem\/#2-code-implementation\" style=\"\">Code Implementation<\/a><\/li><li style=\"\"><a href=\"https:\/\/codeanddebug.in\/blog\/perfect-sum-problem\/#3-code-explanation\" style=\"\">Code Explanation<\/a><\/li><li style=\"\"><a href=\"https:\/\/codeanddebug.in\/blog\/perfect-sum-problem\/#4-time-and-space-complexity\" style=\"\">Time and Space Complexity<\/a><\/li><\/ul><\/li><li style=\"\"><a href=\"https:\/\/codeanddebug.in\/blog\/perfect-sum-problem\/#5-solution-2-memoization-top-down-dp\" style=\"\">Solution 2: Memoization (Top-Down DP)<\/a><ul><li style=\"\"><a href=\"https:\/\/codeanddebug.in\/blog\/perfect-sum-problem\/#6-intuition-and-approach\" style=\"\">Intuition and Approach<\/a><\/li><li style=\"\"><a href=\"https:\/\/codeanddebug.in\/blog\/perfect-sum-problem\/#7-code-implementation\" style=\"\">Code Implementation<\/a><\/li><li style=\"\"><a href=\"https:\/\/codeanddebug.in\/blog\/perfect-sum-problem\/#8-code-explanation\" style=\"\">Code Explanation<\/a><\/li><li style=\"\"><a href=\"https:\/\/codeanddebug.in\/blog\/perfect-sum-problem\/#9-time-and-space-complexity\" style=\"\">Time and Space Complexity<\/a><\/li><\/ul><\/li><li style=\"\"><a href=\"https:\/\/codeanddebug.in\/blog\/perfect-sum-problem\/#10-solution-3-tabulation-bottom-up-dp\" style=\"\">Solution 3: Tabulation (Bottom-Up DP)<\/a><ul><li style=\"\"><a href=\"https:\/\/codeanddebug.in\/blog\/perfect-sum-problem\/#11-intuition-and-approach\" style=\"\">Intuition and Approach<\/a><\/li><li style=\"\"><a href=\"https:\/\/codeanddebug.in\/blog\/perfect-sum-problem\/#12-code-implementation\" style=\"\">Code Implementation<\/a><\/li><li style=\"\"><a href=\"https:\/\/codeanddebug.in\/blog\/perfect-sum-problem\/#13-code-explanation\" style=\"\">Code Explanation<\/a><\/li><li style=\"\"><a href=\"https:\/\/codeanddebug.in\/blog\/perfect-sum-problem\/#14-time-and-space-complexity\" style=\"\">Time and Space Complexity<\/a><\/li><\/ul><\/li><li style=\"\"><a href=\"https:\/\/codeanddebug.in\/blog\/perfect-sum-problem\/#15-solution-4-tabulation-space-optimization\" style=\"\">Solution 4: Tabulation (Space Optimization)<\/a><ul><li style=\"\"><a href=\"https:\/\/codeanddebug.in\/blog\/perfect-sum-problem\/#16-intuition-and-approach\" style=\"\">Intuition and Approach<\/a><\/li><li style=\"\"><a href=\"https:\/\/codeanddebug.in\/blog\/perfect-sum-problem\/#17-code-implementation\" style=\"\">Code Implementation<\/a><\/li><li style=\"\"><a href=\"https:\/\/codeanddebug.in\/blog\/perfect-sum-problem\/#18-code-explanation\" style=\"\">Code Explanation<\/a><\/li><li style=\"\"><a href=\"https:\/\/codeanddebug.in\/blog\/perfect-sum-problem\/#19-time-and-space-complexity\" style=\"\">Time and Space Complexity<\/a><\/li><\/ul><\/li><li style=\"\"><a href=\"https:\/\/codeanddebug.in\/blog\/perfect-sum-problem\/#20-tips-and-edge-cases\" style=\"\">Tips and Edge Cases<\/a><\/li><li style=\"\"><a href=\"https:\/\/codeanddebug.in\/blog\/perfect-sum-problem\/#21-conclusion\" style=\"\">Conclusion<\/a><\/li><\/ul>\n\t\t\t<\/div>\n\t\t<\/div><\/div>\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"0-solution-1-recursion-brute-force\">Solution 1: Recursion (Brute Force)<\/h2>\n\n\n\n<h3 class=\"wp-block-heading\" id=\"1-intuition-and-approach\">Intuition and Approach<\/h3>\n\n\n\n<p>At each index, you can:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li>Pick the current element if it doesn\u2019t exceed the remaining total.<\/li>\n\n\n\n<li>Not pick it.<\/li>\n<\/ul>\n\n\n\n<p>Base case at index 0:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li>If total == 0 and arr == 0 \u2192 2 ways (include\/exclude zero).<\/li>\n\n\n\n<li>Else if total == 0 or arr == total \u2192 1 way.<\/li>\n\n\n\n<li>Otherwise \u2192 0 ways.<\/li>\n<\/ul>\n\n\n\n<p>This explores all subsets and sums counts.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\" id=\"2-code-implementation\">Code Implementation<\/h3>\n\n\n\n<div class=\"wp-block-kevinbatdorf-code-block-pro cbp-has-line-numbers\" data-code-block-pro-font-family=\"Code-Pro-JetBrains-Mono\" style=\"font-size:.875rem;font-family:Code-Pro-JetBrains-Mono,ui-monospace,SFMono-Regular,Menlo,Monaco,Consolas,monospace;--cbp-line-number-color:#D4D4D4;--cbp-line-number-width:calc(2 * 0.6 * .875rem);line-height:1.5rem;--cbp-tab-width:2;tab-size:var(--cbp-tab-width, 2)\"><span style=\"display:block;padding:16px 0 0 16px;margin-bottom:-1px;width:100%;text-align:left;background-color:#1E1E1E\"><svg xmlns=\"http:\/\/www.w3.org\/2000\/svg\" width=\"54\" height=\"14\" viewBox=\"0 0 54 14\"><g fill=\"none\" fill-rule=\"evenodd\" transform=\"translate(1 1)\"><circle cx=\"6\" cy=\"6\" r=\"6\" fill=\"#FF5F56\" stroke=\"#E0443E\" stroke-width=\".5\"><\/circle><circle cx=\"26\" cy=\"6\" r=\"6\" fill=\"#FFBD2E\" stroke=\"#DEA123\" stroke-width=\".5\"><\/circle><circle cx=\"46\" cy=\"6\" r=\"6\" fill=\"#27C93F\" stroke=\"#1AAB29\" stroke-width=\".5\"><\/circle><\/g><\/svg><\/span><span role=\"button\" tabindex=\"0\" data-code=\"class Solution:\n    def solve(self, index, total, arr):\n        # Base case at first element (handle zero carefully)\n        if index == 0:\n            if total == 0 and arr == 0:\n                return 2\n            if total == 0 or arr == total:\n                return 1\n            return 0\n        # Try pick if feasible, else 0\n        if arr[index] &gt; total:\n            pick = 0\n        else:\n            pick = self.solve(index - 1, total - arr[index], arr)\n        # Not pick path\n        not_pick = self.solve(index - 1, total, arr)\n        # Total ways = pick + not_pick\n        return pick + not_pick\n\n    def perfectSum(self, arr, target):\n        n = len(arr)\n        return self.solve(n - 1, target, arr)\" style=\"color:#D4D4D4;display:none\" aria-label=\"Copy\" class=\"code-block-pro-copy-button\"><svg xmlns=\"http:\/\/www.w3.org\/2000\/svg\" style=\"width:24px;height:24px\" fill=\"none\" viewBox=\"0 0 24 24\" stroke=\"currentColor\" stroke-width=\"2\"><path class=\"with-check\" stroke-linecap=\"round\" stroke-linejoin=\"round\" d=\"M9 5H7a2 2 0 00-2 2v12a2 2 0 002 2h10a2 2 0 002-2V7a2 2 0 00-2-2h-2M9 5a2 2 0 002 2h2a2 2 0 002-2M9 5a2 2 0 012-2h2a2 2 0 012 2m-6 9l2 2 4-4\"><\/path><path class=\"without-check\" stroke-linecap=\"round\" stroke-linejoin=\"round\" d=\"M9 5H7a2 2 0 00-2 2v12a2 2 0 002 2h10a2 2 0 002-2V7a2 2 0 00-2-2h-2M9 5a2 2 0 002 2h2a2 2 0 002-2M9 5a2 2 0 012-2h2a2 2 0 012 2\"><\/path><\/svg><\/span><pre class=\"shiki dark-plus\" style=\"background-color: #1E1E1E\" tabindex=\"0\"><code><span class=\"line\"><span style=\"color: #569CD6\">class<\/span><span style=\"color: #D4D4D4\"> <\/span><span style=\"color: #4EC9B0\">Solution<\/span><span style=\"color: #D4D4D4\">:<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">    <\/span><span style=\"color: #569CD6\">def<\/span><span style=\"color: #D4D4D4\"> <\/span><span style=\"color: #DCDCAA\">solve<\/span><span style=\"color: #D4D4D4\">(<\/span><span style=\"color: #9CDCFE\">self<\/span><span style=\"color: #D4D4D4\">, <\/span><span style=\"color: #9CDCFE\">index<\/span><span style=\"color: #D4D4D4\">, <\/span><span style=\"color: #9CDCFE\">total<\/span><span style=\"color: #D4D4D4\">, <\/span><span style=\"color: #9CDCFE\">arr<\/span><span style=\"color: #D4D4D4\">):<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><span style=\"color: #6A9955\"># Base case at first element (handle zero carefully)<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><span style=\"color: #C586C0\">if<\/span><span style=\"color: #D4D4D4\"> index == <\/span><span style=\"color: #B5CEA8\">0<\/span><span style=\"color: #D4D4D4\">:<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">            <\/span><span style=\"color: #C586C0\">if<\/span><span style=\"color: #D4D4D4\"> total == <\/span><span style=\"color: #B5CEA8\">0<\/span><span style=\"color: #D4D4D4\"> <\/span><span style=\"color: #569CD6\">and<\/span><span style=\"color: #D4D4D4\"> arr == <\/span><span style=\"color: #B5CEA8\">0<\/span><span style=\"color: #D4D4D4\">:<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">                <\/span><span style=\"color: #C586C0\">return<\/span><span style=\"color: #D4D4D4\"> <\/span><span style=\"color: #B5CEA8\">2<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">            <\/span><span style=\"color: #C586C0\">if<\/span><span style=\"color: #D4D4D4\"> total == <\/span><span style=\"color: #B5CEA8\">0<\/span><span style=\"color: #D4D4D4\"> <\/span><span style=\"color: #569CD6\">or<\/span><span style=\"color: #D4D4D4\"> arr == total:<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">                <\/span><span style=\"color: #C586C0\">return<\/span><span style=\"color: #D4D4D4\"> <\/span><span style=\"color: #B5CEA8\">1<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">            <\/span><span style=\"color: #C586C0\">return<\/span><span style=\"color: #D4D4D4\"> <\/span><span style=\"color: #B5CEA8\">0<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><span style=\"color: #6A9955\"># Try pick if feasible, else 0<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><span style=\"color: #C586C0\">if<\/span><span style=\"color: #D4D4D4\"> arr[index] &gt; total:<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">            pick = <\/span><span style=\"color: #B5CEA8\">0<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><span style=\"color: #C586C0\">else<\/span><span style=\"color: #D4D4D4\">:<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">            pick = <\/span><span style=\"color: #569CD6\">self<\/span><span style=\"color: #D4D4D4\">.solve(index - <\/span><span style=\"color: #B5CEA8\">1<\/span><span style=\"color: #D4D4D4\">, total - arr[index], arr)<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><span style=\"color: #6A9955\"># Not pick path<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        not_pick = <\/span><span style=\"color: #569CD6\">self<\/span><span style=\"color: #D4D4D4\">.solve(index - <\/span><span style=\"color: #B5CEA8\">1<\/span><span style=\"color: #D4D4D4\">, total, arr)<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><span style=\"color: #6A9955\"># Total ways = pick + not_pick<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><span style=\"color: #C586C0\">return<\/span><span style=\"color: #D4D4D4\"> pick + not_pick<\/span><\/span>\n<span class=\"line\"><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">    <\/span><span style=\"color: #569CD6\">def<\/span><span style=\"color: #D4D4D4\"> <\/span><span style=\"color: #DCDCAA\">perfectSum<\/span><span style=\"color: #D4D4D4\">(<\/span><span style=\"color: #9CDCFE\">self<\/span><span style=\"color: #D4D4D4\">, <\/span><span style=\"color: #9CDCFE\">arr<\/span><span style=\"color: #D4D4D4\">, <\/span><span style=\"color: #9CDCFE\">target<\/span><span style=\"color: #D4D4D4\">):<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        n = <\/span><span style=\"color: #DCDCAA\">len<\/span><span style=\"color: #D4D4D4\">(arr)<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><span style=\"color: #C586C0\">return<\/span><span style=\"color: #D4D4D4\"> <\/span><span style=\"color: #569CD6\">self<\/span><span style=\"color: #D4D4D4\">.solve(n - <\/span><span style=\"color: #B5CEA8\">1<\/span><span style=\"color: #D4D4D4\">, target, arr)<\/span><\/span><\/code><\/pre><\/div>\n\n\n\n<h3 class=\"wp-block-heading\" id=\"3-code-explanation\">Code Explanation<\/h3>\n\n\n\n<ul class=\"wp-block-list\">\n<li>The base handles all zero\/target relationships at index 0 to ensure correct counting.<\/li>\n\n\n\n<li>For each index, add ways from picking and not picking when possible.<\/li>\n\n\n\n<li>Returns total number of valid subsets.<\/li>\n<\/ul>\n\n\n\n<h3 class=\"wp-block-heading\" id=\"4-time-and-space-complexity\">Time and Space Complexity<\/h3>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>Time<\/strong>: Exponential (O(2^n)) due to exploring all subsets.<\/li>\n\n\n\n<li><strong>Space<\/strong>: O(n) recursion depth.<\/li>\n<\/ul>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"5-solution-2-memoization-top-down-dp\">Solution 2: Memoization (Top-Down DP)<\/h2>\n\n\n\n<h3 class=\"wp-block-heading\" id=\"6-intuition-and-approach\">Intuition and Approach<\/h3>\n\n\n\n<p>Same recursion, but cache results for (index, total) to avoid recomputation.<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li>dp[index][total] stores number of ways to reach sum total using elements up to index.<\/li>\n<\/ul>\n\n\n\n<h3 class=\"wp-block-heading\" id=\"7-code-implementation\">Code Implementation<\/h3>\n\n\n\n<div class=\"wp-block-kevinbatdorf-code-block-pro cbp-has-line-numbers\" data-code-block-pro-font-family=\"Code-Pro-JetBrains-Mono\" style=\"font-size:.875rem;font-family:Code-Pro-JetBrains-Mono,ui-monospace,SFMono-Regular,Menlo,Monaco,Consolas,monospace;--cbp-line-number-color:#D4D4D4;--cbp-line-number-width:calc(2 * 0.6 * .875rem);line-height:1.5rem;--cbp-tab-width:2;tab-size:var(--cbp-tab-width, 2)\"><span style=\"display:block;padding:16px 0 0 16px;margin-bottom:-1px;width:100%;text-align:left;background-color:#1E1E1E\"><svg xmlns=\"http:\/\/www.w3.org\/2000\/svg\" width=\"54\" height=\"14\" viewBox=\"0 0 54 14\"><g fill=\"none\" fill-rule=\"evenodd\" transform=\"translate(1 1)\"><circle cx=\"6\" cy=\"6\" r=\"6\" fill=\"#FF5F56\" stroke=\"#E0443E\" stroke-width=\".5\"><\/circle><circle cx=\"26\" cy=\"6\" r=\"6\" fill=\"#FFBD2E\" stroke=\"#DEA123\" stroke-width=\".5\"><\/circle><circle cx=\"46\" cy=\"6\" r=\"6\" fill=\"#27C93F\" stroke=\"#1AAB29\" stroke-width=\".5\"><\/circle><\/g><\/svg><\/span><span role=\"button\" tabindex=\"0\" data-code=\"class Solution:\n    def solve(self, index, total, arr, dp):\n        # Base case at first element\n        if index == 0:\n            if total == 0 and arr == 0:\n                return 2\n            if total == 0 or arr == total:\n                return 1\n            return 0\n        # Return cached value if present\n        if dp[index][total] != -1:\n            return dp[index][total]\n        # Pick if feasible\n        if arr[index] &gt; total:\n            pick = 0\n        else:\n            pick = self.solve(index - 1, total - arr[index], arr, dp)\n        # Not pick\n        not_pick = self.solve(index - 1, total, arr, dp)\n        dp[index][total] = pick + not_pick\n        return dp[index][total]\n\n    def perfectSum(self, arr, target):\n        n = len(arr)\n        dp = [[-1 for _ in range(target + 1)] for _ in range(n)]\n        return self.solve(n - 1, target, arr, dp)\" style=\"color:#D4D4D4;display:none\" aria-label=\"Copy\" class=\"code-block-pro-copy-button\"><svg xmlns=\"http:\/\/www.w3.org\/2000\/svg\" style=\"width:24px;height:24px\" fill=\"none\" viewBox=\"0 0 24 24\" stroke=\"currentColor\" stroke-width=\"2\"><path class=\"with-check\" stroke-linecap=\"round\" stroke-linejoin=\"round\" d=\"M9 5H7a2 2 0 00-2 2v12a2 2 0 002 2h10a2 2 0 002-2V7a2 2 0 00-2-2h-2M9 5a2 2 0 002 2h2a2 2 0 002-2M9 5a2 2 0 012-2h2a2 2 0 012 2m-6 9l2 2 4-4\"><\/path><path class=\"without-check\" stroke-linecap=\"round\" stroke-linejoin=\"round\" d=\"M9 5H7a2 2 0 00-2 2v12a2 2 0 002 2h10a2 2 0 002-2V7a2 2 0 00-2-2h-2M9 5a2 2 0 002 2h2a2 2 0 002-2M9 5a2 2 0 012-2h2a2 2 0 012 2\"><\/path><\/svg><\/span><pre class=\"shiki dark-plus\" style=\"background-color: #1E1E1E\" tabindex=\"0\"><code><span class=\"line\"><span style=\"color: #569CD6\">class<\/span><span style=\"color: #D4D4D4\"> <\/span><span style=\"color: #4EC9B0\">Solution<\/span><span style=\"color: #D4D4D4\">:<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">    <\/span><span style=\"color: #569CD6\">def<\/span><span style=\"color: #D4D4D4\"> <\/span><span style=\"color: #DCDCAA\">solve<\/span><span style=\"color: #D4D4D4\">(<\/span><span style=\"color: #9CDCFE\">self<\/span><span style=\"color: #D4D4D4\">, <\/span><span style=\"color: #9CDCFE\">index<\/span><span style=\"color: #D4D4D4\">, <\/span><span style=\"color: #9CDCFE\">total<\/span><span style=\"color: #D4D4D4\">, <\/span><span style=\"color: #9CDCFE\">arr<\/span><span style=\"color: #D4D4D4\">, <\/span><span style=\"color: #9CDCFE\">dp<\/span><span style=\"color: #D4D4D4\">):<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><span style=\"color: #6A9955\"># Base case at first element<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><span style=\"color: #C586C0\">if<\/span><span style=\"color: #D4D4D4\"> index == <\/span><span style=\"color: #B5CEA8\">0<\/span><span style=\"color: #D4D4D4\">:<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">            <\/span><span style=\"color: #C586C0\">if<\/span><span style=\"color: #D4D4D4\"> total == <\/span><span style=\"color: #B5CEA8\">0<\/span><span style=\"color: #D4D4D4\"> <\/span><span style=\"color: #569CD6\">and<\/span><span style=\"color: #D4D4D4\"> arr == <\/span><span style=\"color: #B5CEA8\">0<\/span><span style=\"color: #D4D4D4\">:<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">                <\/span><span style=\"color: #C586C0\">return<\/span><span style=\"color: #D4D4D4\"> <\/span><span style=\"color: #B5CEA8\">2<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">            <\/span><span style=\"color: #C586C0\">if<\/span><span style=\"color: #D4D4D4\"> total == <\/span><span style=\"color: #B5CEA8\">0<\/span><span style=\"color: #D4D4D4\"> <\/span><span style=\"color: #569CD6\">or<\/span><span style=\"color: #D4D4D4\"> arr == total:<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">                <\/span><span style=\"color: #C586C0\">return<\/span><span style=\"color: #D4D4D4\"> <\/span><span style=\"color: #B5CEA8\">1<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">            <\/span><span style=\"color: #C586C0\">return<\/span><span style=\"color: #D4D4D4\"> <\/span><span style=\"color: #B5CEA8\">0<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><span style=\"color: #6A9955\"># Return cached value if present<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><span style=\"color: #C586C0\">if<\/span><span style=\"color: #D4D4D4\"> dp[index][total] != -<\/span><span style=\"color: #B5CEA8\">1<\/span><span style=\"color: #D4D4D4\">:<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">            <\/span><span style=\"color: #C586C0\">return<\/span><span style=\"color: #D4D4D4\"> dp[index][total]<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><span style=\"color: #6A9955\"># Pick if feasible<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><span style=\"color: #C586C0\">if<\/span><span style=\"color: #D4D4D4\"> arr[index] &gt; total:<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">            pick = <\/span><span style=\"color: #B5CEA8\">0<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><span style=\"color: #C586C0\">else<\/span><span style=\"color: #D4D4D4\">:<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">            pick = <\/span><span style=\"color: #569CD6\">self<\/span><span style=\"color: #D4D4D4\">.solve(index - <\/span><span style=\"color: #B5CEA8\">1<\/span><span style=\"color: #D4D4D4\">, total - arr[index], arr, dp)<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><span style=\"color: #6A9955\"># Not pick<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        not_pick = <\/span><span style=\"color: #569CD6\">self<\/span><span style=\"color: #D4D4D4\">.solve(index - <\/span><span style=\"color: #B5CEA8\">1<\/span><span style=\"color: #D4D4D4\">, total, arr, dp)<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        dp[index][total] = pick + not_pick<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><span style=\"color: #C586C0\">return<\/span><span style=\"color: #D4D4D4\"> dp[index][total]<\/span><\/span>\n<span class=\"line\"><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">    <\/span><span style=\"color: #569CD6\">def<\/span><span style=\"color: #D4D4D4\"> <\/span><span style=\"color: #DCDCAA\">perfectSum<\/span><span style=\"color: #D4D4D4\">(<\/span><span style=\"color: #9CDCFE\">self<\/span><span style=\"color: #D4D4D4\">, <\/span><span style=\"color: #9CDCFE\">arr<\/span><span style=\"color: #D4D4D4\">, <\/span><span style=\"color: #9CDCFE\">target<\/span><span style=\"color: #D4D4D4\">):<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        n = <\/span><span style=\"color: #DCDCAA\">len<\/span><span style=\"color: #D4D4D4\">(arr)<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        dp = [[-<\/span><span style=\"color: #B5CEA8\">1<\/span><span style=\"color: #D4D4D4\"> <\/span><span style=\"color: #C586C0\">for<\/span><span style=\"color: #D4D4D4\"> _ <\/span><span style=\"color: #C586C0\">in<\/span><span style=\"color: #D4D4D4\"> <\/span><span style=\"color: #DCDCAA\">range<\/span><span style=\"color: #D4D4D4\">(target + <\/span><span style=\"color: #B5CEA8\">1<\/span><span style=\"color: #D4D4D4\">)] <\/span><span style=\"color: #C586C0\">for<\/span><span style=\"color: #D4D4D4\"> _ <\/span><span style=\"color: #C586C0\">in<\/span><span style=\"color: #D4D4D4\"> <\/span><span style=\"color: #DCDCAA\">range<\/span><span style=\"color: #D4D4D4\">(n)]<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><span style=\"color: #C586C0\">return<\/span><span style=\"color: #D4D4D4\"> <\/span><span style=\"color: #569CD6\">self<\/span><span style=\"color: #D4D4D4\">.solve(n - <\/span><span style=\"color: #B5CEA8\">1<\/span><span style=\"color: #D4D4D4\">, target, arr, dp)<\/span><\/span><\/code><\/pre><\/div>\n\n\n\n<h3 class=\"wp-block-heading\" id=\"8-code-explanation\">Code Explanation<\/h3>\n\n\n\n<ul class=\"wp-block-list\">\n<li>Adds memoization to cut exponential recomputation.<\/li>\n\n\n\n<li>The base case logic with zero handling remains identical for correctness.<\/li>\n<\/ul>\n\n\n\n<h3 class=\"wp-block-heading\" id=\"9-time-and-space-complexity\">Time and Space Complexity<\/h3>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>Time<\/strong>: O(n \u00d7 target) states, O(1) per state \u2192 O(n \u00d7 target).<\/li>\n\n\n\n<li><strong>Space<\/strong>: O(n \u00d7 target) for dp + O(n) recursion stack.<\/li>\n<\/ul>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"10-solution-3-tabulation-bottom-up-dp\">Solution 3: Tabulation (Bottom-Up DP)<\/h2>\n\n\n\n<h3 class=\"wp-block-heading\" id=\"11-intuition-and-approach\">Intuition and Approach<\/h3>\n\n\n\n<p>Build dp iteratively:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li>dp[i][t] = number of ways to make sum t using elements up to index i.<\/li>\n\n\n\n<li>Initialize row 0 carefully to account for zero.<\/li>\n\n\n\n<li>Transition: dp[i][t] = dp[i-1][t] (not pick) + (dp[i-1][t &#8211; arr[i]] if arr[i] \u2264 t else 0).<\/li>\n<\/ul>\n\n\n\n<h3 class=\"wp-block-heading\" id=\"12-code-implementation\">Code Implementation<\/h3>\n\n\n\n<div class=\"wp-block-kevinbatdorf-code-block-pro cbp-has-line-numbers\" data-code-block-pro-font-family=\"Code-Pro-JetBrains-Mono\" style=\"font-size:.875rem;font-family:Code-Pro-JetBrains-Mono,ui-monospace,SFMono-Regular,Menlo,Monaco,Consolas,monospace;--cbp-line-number-color:#D4D4D4;--cbp-line-number-width:calc(2 * 0.6 * .875rem);line-height:1.5rem;--cbp-tab-width:2;tab-size:var(--cbp-tab-width, 2)\"><span style=\"display:block;padding:16px 0 0 16px;margin-bottom:-1px;width:100%;text-align:left;background-color:#1E1E1E\"><svg xmlns=\"http:\/\/www.w3.org\/2000\/svg\" width=\"54\" height=\"14\" viewBox=\"0 0 54 14\"><g fill=\"none\" fill-rule=\"evenodd\" transform=\"translate(1 1)\"><circle cx=\"6\" cy=\"6\" r=\"6\" fill=\"#FF5F56\" stroke=\"#E0443E\" stroke-width=\".5\"><\/circle><circle cx=\"26\" cy=\"6\" r=\"6\" fill=\"#FFBD2E\" stroke=\"#DEA123\" stroke-width=\".5\"><\/circle><circle cx=\"46\" cy=\"6\" r=\"6\" fill=\"#27C93F\" stroke=\"#1AAB29\" stroke-width=\".5\"><\/circle><\/g><\/svg><\/span><span role=\"button\" tabindex=\"0\" data-code=\"class Solution:\n    def perfectSum(self, arr, target):\n        n = len(arr)\n        dp = [[0 for _ in range(0, target + 1)] for _ in range(n)]\n        # Base init for index 0 (handle zero specially)\n        if arr[0] == 0:\n            dp = 2\n        else:\n            dp = 1\n            if arr &lt;= target:\n                dp[arr] = 1\n        # Fill table\n        for index in range(1, n):\n            for total in range(0, target + 1):\n                if arr[index] &gt; total:\n                    pick = 0\n                else:\n                    pick = dp[index - 1][total - arr[index]]\n                not_pick = dp[index - 1][total]\n                dp[index][total] = pick + not_pick\n        return dp[n - 1][target]\" style=\"color:#D4D4D4;display:none\" aria-label=\"Copy\" class=\"code-block-pro-copy-button\"><svg xmlns=\"http:\/\/www.w3.org\/2000\/svg\" style=\"width:24px;height:24px\" fill=\"none\" viewBox=\"0 0 24 24\" stroke=\"currentColor\" stroke-width=\"2\"><path class=\"with-check\" stroke-linecap=\"round\" stroke-linejoin=\"round\" d=\"M9 5H7a2 2 0 00-2 2v12a2 2 0 002 2h10a2 2 0 002-2V7a2 2 0 00-2-2h-2M9 5a2 2 0 002 2h2a2 2 0 002-2M9 5a2 2 0 012-2h2a2 2 0 012 2m-6 9l2 2 4-4\"><\/path><path class=\"without-check\" stroke-linecap=\"round\" stroke-linejoin=\"round\" d=\"M9 5H7a2 2 0 00-2 2v12a2 2 0 002 2h10a2 2 0 002-2V7a2 2 0 00-2-2h-2M9 5a2 2 0 002 2h2a2 2 0 002-2M9 5a2 2 0 012-2h2a2 2 0 012 2\"><\/path><\/svg><\/span><pre class=\"shiki dark-plus\" style=\"background-color: #1E1E1E\" tabindex=\"0\"><code><span class=\"line\"><span style=\"color: #569CD6\">class<\/span><span style=\"color: #D4D4D4\"> <\/span><span style=\"color: #4EC9B0\">Solution<\/span><span style=\"color: #D4D4D4\">:<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">    <\/span><span style=\"color: #569CD6\">def<\/span><span style=\"color: #D4D4D4\"> <\/span><span style=\"color: #DCDCAA\">perfectSum<\/span><span style=\"color: #D4D4D4\">(<\/span><span style=\"color: #9CDCFE\">self<\/span><span style=\"color: #D4D4D4\">, <\/span><span style=\"color: #9CDCFE\">arr<\/span><span style=\"color: #D4D4D4\">, <\/span><span style=\"color: #9CDCFE\">target<\/span><span style=\"color: #D4D4D4\">):<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        n = <\/span><span style=\"color: #DCDCAA\">len<\/span><span style=\"color: #D4D4D4\">(arr)<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        dp = [[<\/span><span style=\"color: #B5CEA8\">0<\/span><span style=\"color: #D4D4D4\"> <\/span><span style=\"color: #C586C0\">for<\/span><span style=\"color: #D4D4D4\"> _ <\/span><span style=\"color: #C586C0\">in<\/span><span style=\"color: #D4D4D4\"> <\/span><span style=\"color: #DCDCAA\">range<\/span><span style=\"color: #D4D4D4\">(<\/span><span style=\"color: #B5CEA8\">0<\/span><span style=\"color: #D4D4D4\">, target + <\/span><span style=\"color: #B5CEA8\">1<\/span><span style=\"color: #D4D4D4\">)] <\/span><span style=\"color: #C586C0\">for<\/span><span style=\"color: #D4D4D4\"> _ <\/span><span style=\"color: #C586C0\">in<\/span><span style=\"color: #D4D4D4\"> <\/span><span style=\"color: #DCDCAA\">range<\/span><span style=\"color: #D4D4D4\">(n)]<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><span style=\"color: #6A9955\"># Base init for index 0 (handle zero specially)<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><span style=\"color: #C586C0\">if<\/span><span style=\"color: #D4D4D4\"> arr[<\/span><span style=\"color: #B5CEA8\">0<\/span><span style=\"color: #D4D4D4\">] == <\/span><span style=\"color: #B5CEA8\">0<\/span><span style=\"color: #D4D4D4\">:<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">            dp = <\/span><span style=\"color: #B5CEA8\">2<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><span style=\"color: #C586C0\">else<\/span><span style=\"color: #D4D4D4\">:<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">            dp = <\/span><span style=\"color: #B5CEA8\">1<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">            <\/span><span style=\"color: #C586C0\">if<\/span><span style=\"color: #D4D4D4\"> arr &lt;= target:<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">                dp[arr] = <\/span><span style=\"color: #B5CEA8\">1<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><span style=\"color: #6A9955\"># Fill table<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><span style=\"color: #C586C0\">for<\/span><span style=\"color: #D4D4D4\"> index <\/span><span style=\"color: #C586C0\">in<\/span><span style=\"color: #D4D4D4\"> <\/span><span style=\"color: #DCDCAA\">range<\/span><span style=\"color: #D4D4D4\">(<\/span><span style=\"color: #B5CEA8\">1<\/span><span style=\"color: #D4D4D4\">, n):<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">            <\/span><span style=\"color: #C586C0\">for<\/span><span style=\"color: #D4D4D4\"> total <\/span><span style=\"color: #C586C0\">in<\/span><span style=\"color: #D4D4D4\"> <\/span><span style=\"color: #DCDCAA\">range<\/span><span style=\"color: #D4D4D4\">(<\/span><span style=\"color: #B5CEA8\">0<\/span><span style=\"color: #D4D4D4\">, target + <\/span><span style=\"color: #B5CEA8\">1<\/span><span style=\"color: #D4D4D4\">):<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">                <\/span><span style=\"color: #C586C0\">if<\/span><span style=\"color: #D4D4D4\"> arr[index] &gt; total:<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">                    pick = <\/span><span style=\"color: #B5CEA8\">0<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">                <\/span><span style=\"color: #C586C0\">else<\/span><span style=\"color: #D4D4D4\">:<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">                    pick = dp[index - <\/span><span style=\"color: #B5CEA8\">1<\/span><span style=\"color: #D4D4D4\">][total - arr[index]]<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">                not_pick = dp[index - <\/span><span style=\"color: #B5CEA8\">1<\/span><span style=\"color: #D4D4D4\">][total]<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">                dp[index][total] = pick + not_pick<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><span style=\"color: #C586C0\">return<\/span><span style=\"color: #D4D4D4\"> dp[n - <\/span><span style=\"color: #B5CEA8\">1<\/span><span style=\"color: #D4D4D4\">][target]<\/span><\/span><\/code><\/pre><\/div>\n\n\n\n<h3 class=\"wp-block-heading\" id=\"13-code-explanation\">Code Explanation<\/h3>\n\n\n\n<ul class=\"wp-block-list\">\n<li>Base initialization:\n<ul class=\"wp-block-list\">\n<li>If arr == 0 \u2192 dp = 2 (include\/exclude zero).<\/li>\n\n\n\n<li>Else dp = 1, and if arr \u2264 target, dp[arr] = 1.<\/li>\n<\/ul>\n<\/li>\n\n\n\n<li>Transitions count ways by adding pick and not_pick counts.<\/li>\n<\/ul>\n\n\n\n<h3 class=\"wp-block-heading\" id=\"14-time-and-space-complexity\">Time and Space Complexity<\/h3>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>Time: <\/strong>O(n \u00d7 target)<\/li>\n\n\n\n<li><strong>Space: <\/strong>O(n \u00d7 target)<\/li>\n<\/ul>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"15-solution-4-tabulation-space-optimization\">Solution 4: Tabulation (Space Optimization)<\/h2>\n\n\n\n<h3 class=\"wp-block-heading\" id=\"16-intuition-and-approach\">Intuition and Approach<\/h3>\n\n\n\n<p>Use two 1D arrays (prev and curr) since each row depends only on previous row.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\" id=\"17-code-implementation\">Code Implementation<\/h3>\n\n\n\n<div class=\"wp-block-kevinbatdorf-code-block-pro cbp-has-line-numbers\" data-code-block-pro-font-family=\"Code-Pro-JetBrains-Mono\" style=\"font-size:.875rem;font-family:Code-Pro-JetBrains-Mono,ui-monospace,SFMono-Regular,Menlo,Monaco,Consolas,monospace;--cbp-line-number-color:#D4D4D4;--cbp-line-number-width:calc(2 * 0.6 * .875rem);line-height:1.5rem;--cbp-tab-width:2;tab-size:var(--cbp-tab-width, 2)\"><span style=\"display:block;padding:16px 0 0 16px;margin-bottom:-1px;width:100%;text-align:left;background-color:#1E1E1E\"><svg xmlns=\"http:\/\/www.w3.org\/2000\/svg\" width=\"54\" height=\"14\" viewBox=\"0 0 54 14\"><g fill=\"none\" fill-rule=\"evenodd\" transform=\"translate(1 1)\"><circle cx=\"6\" cy=\"6\" r=\"6\" fill=\"#FF5F56\" stroke=\"#E0443E\" stroke-width=\".5\"><\/circle><circle cx=\"26\" cy=\"6\" r=\"6\" fill=\"#FFBD2E\" stroke=\"#DEA123\" stroke-width=\".5\"><\/circle><circle cx=\"46\" cy=\"6\" r=\"6\" fill=\"#27C93F\" stroke=\"#1AAB29\" stroke-width=\".5\"><\/circle><\/g><\/svg><\/span><span role=\"button\" tabindex=\"0\" data-code=\"class Solution:\n    def perfectSum(self, arr, target):\n        n = len(arr)\n        prev = [0 for _ in range(0, target + 1)]\n        # Base init for index 0\n        if arr == 0:\n            prev = 2\n        else:\n            prev = 1\n            if arr &lt;= target:\n                prev[arr] = 1\n        # Build with rolling arrays\n        for index in range(1, n):\n            curr = [0 for _ in range(0, target + 1)]\n            for total in range(0, target + 1):\n                if arr[index] &gt; total:\n                    pick = 0\n                else:\n                    pick = prev[total - arr[index]]\n                not_pick = prev[total]\n                curr[total] = pick + not_pick\n            prev = curr\n        return prev[target]\" style=\"color:#D4D4D4;display:none\" aria-label=\"Copy\" class=\"code-block-pro-copy-button\"><svg xmlns=\"http:\/\/www.w3.org\/2000\/svg\" style=\"width:24px;height:24px\" fill=\"none\" viewBox=\"0 0 24 24\" stroke=\"currentColor\" stroke-width=\"2\"><path class=\"with-check\" stroke-linecap=\"round\" stroke-linejoin=\"round\" d=\"M9 5H7a2 2 0 00-2 2v12a2 2 0 002 2h10a2 2 0 002-2V7a2 2 0 00-2-2h-2M9 5a2 2 0 002 2h2a2 2 0 002-2M9 5a2 2 0 012-2h2a2 2 0 012 2m-6 9l2 2 4-4\"><\/path><path class=\"without-check\" stroke-linecap=\"round\" stroke-linejoin=\"round\" d=\"M9 5H7a2 2 0 00-2 2v12a2 2 0 002 2h10a2 2 0 002-2V7a2 2 0 00-2-2h-2M9 5a2 2 0 002 2h2a2 2 0 002-2M9 5a2 2 0 012-2h2a2 2 0 012 2\"><\/path><\/svg><\/span><pre class=\"shiki dark-plus\" style=\"background-color: #1E1E1E\" tabindex=\"0\"><code><span class=\"line\"><span style=\"color: #569CD6\">class<\/span><span style=\"color: #D4D4D4\"> <\/span><span style=\"color: #4EC9B0\">Solution<\/span><span style=\"color: #D4D4D4\">:<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">    <\/span><span style=\"color: #569CD6\">def<\/span><span style=\"color: #D4D4D4\"> <\/span><span style=\"color: #DCDCAA\">perfectSum<\/span><span style=\"color: #D4D4D4\">(<\/span><span style=\"color: #9CDCFE\">self<\/span><span style=\"color: #D4D4D4\">, <\/span><span style=\"color: #9CDCFE\">arr<\/span><span style=\"color: #D4D4D4\">, <\/span><span style=\"color: #9CDCFE\">target<\/span><span style=\"color: #D4D4D4\">):<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        n = <\/span><span style=\"color: #DCDCAA\">len<\/span><span style=\"color: #D4D4D4\">(arr)<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        prev = [<\/span><span style=\"color: #B5CEA8\">0<\/span><span style=\"color: #D4D4D4\"> <\/span><span style=\"color: #C586C0\">for<\/span><span style=\"color: #D4D4D4\"> _ <\/span><span style=\"color: #C586C0\">in<\/span><span style=\"color: #D4D4D4\"> <\/span><span style=\"color: #DCDCAA\">range<\/span><span style=\"color: #D4D4D4\">(<\/span><span style=\"color: #B5CEA8\">0<\/span><span style=\"color: #D4D4D4\">, target + <\/span><span style=\"color: #B5CEA8\">1<\/span><span style=\"color: #D4D4D4\">)]<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><span style=\"color: #6A9955\"># Base init for index 0<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><span style=\"color: #C586C0\">if<\/span><span style=\"color: #D4D4D4\"> arr == <\/span><span style=\"color: #B5CEA8\">0<\/span><span style=\"color: #D4D4D4\">:<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">            prev = <\/span><span style=\"color: #B5CEA8\">2<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><span style=\"color: #C586C0\">else<\/span><span style=\"color: #D4D4D4\">:<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">            prev = <\/span><span style=\"color: #B5CEA8\">1<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">            <\/span><span style=\"color: #C586C0\">if<\/span><span style=\"color: #D4D4D4\"> arr &lt;= target:<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">                prev[arr] = <\/span><span style=\"color: #B5CEA8\">1<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><span style=\"color: #6A9955\"># Build with rolling arrays<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><span style=\"color: #C586C0\">for<\/span><span style=\"color: #D4D4D4\"> index <\/span><span style=\"color: #C586C0\">in<\/span><span style=\"color: #D4D4D4\"> <\/span><span style=\"color: #DCDCAA\">range<\/span><span style=\"color: #D4D4D4\">(<\/span><span style=\"color: #B5CEA8\">1<\/span><span style=\"color: #D4D4D4\">, n):<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">            curr = [<\/span><span style=\"color: #B5CEA8\">0<\/span><span style=\"color: #D4D4D4\"> <\/span><span style=\"color: #C586C0\">for<\/span><span style=\"color: #D4D4D4\"> _ <\/span><span style=\"color: #C586C0\">in<\/span><span style=\"color: #D4D4D4\"> <\/span><span style=\"color: #DCDCAA\">range<\/span><span style=\"color: #D4D4D4\">(<\/span><span style=\"color: #B5CEA8\">0<\/span><span style=\"color: #D4D4D4\">, target + <\/span><span style=\"color: #B5CEA8\">1<\/span><span style=\"color: #D4D4D4\">)]<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">            <\/span><span style=\"color: #C586C0\">for<\/span><span style=\"color: #D4D4D4\"> total <\/span><span style=\"color: #C586C0\">in<\/span><span style=\"color: #D4D4D4\"> <\/span><span style=\"color: #DCDCAA\">range<\/span><span style=\"color: #D4D4D4\">(<\/span><span style=\"color: #B5CEA8\">0<\/span><span style=\"color: #D4D4D4\">, target + <\/span><span style=\"color: #B5CEA8\">1<\/span><span style=\"color: #D4D4D4\">):<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">                <\/span><span style=\"color: #C586C0\">if<\/span><span style=\"color: #D4D4D4\"> arr[index] &gt; total:<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">                    pick = <\/span><span style=\"color: #B5CEA8\">0<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">                <\/span><span style=\"color: #C586C0\">else<\/span><span style=\"color: #D4D4D4\">:<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">                    pick = prev[total - arr[index]]<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">                not_pick = prev[total]<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">                curr[total] = pick + not_pick<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">            prev = curr<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><span style=\"color: #C586C0\">return<\/span><span style=\"color: #D4D4D4\"> prev[target]<\/span><\/span><\/code><\/pre><\/div>\n\n\n\n<h3 class=\"wp-block-heading\" id=\"18-code-explanation\">Code Explanation<\/h3>\n\n\n\n<ul class=\"wp-block-list\">\n<li>Mirrors the full table logic with O(target) space.<\/li>\n\n\n\n<li>Base and transitions remain identical, ensuring correctness with zeros.<\/li>\n<\/ul>\n\n\n\n<h3 class=\"wp-block-heading\" id=\"19-time-and-space-complexity\">Time and Space Complexity<\/h3>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>Time<\/strong>: O(n \u00d7 target)<\/li>\n\n\n\n<li><strong>Space:<\/strong> O(target)<\/li>\n<\/ul>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"20-tips-and-edge-cases\">Tips and Edge Cases<\/h2>\n\n\n\n<ul class=\"wp-block-list\">\n<li>Zeros: The base case and first-row initialization must treat arr == 0 specially, as it doubles the count for sum 0 at index 0.<\/li>\n\n\n\n<li>Large counts: Depending on platform constraints, you might need modulus; your current code returns raw counts, which is typically fine unless specified otherwise.<\/li>\n\n\n\n<li>Order doesn\u2019t matter: We count subsets (combinations), not sequences, so the DP uses each element at most once.<\/li>\n<\/ul>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"21-conclusion\">Conclusion<\/h2>\n\n\n\n<p>The Perfect Sum Problem is a counting variant of Subset Sum where careful handling of zeros is crucial. Starting from recursion for clarity, you can optimize to memoization and tabulation; the space-optimized tabulation is the most interview-friendly with O(n \u00d7 target) time and O(target) space while preserving correctness.<\/p>\n\n\n\n<div class=\"wp-block-buttons is-content-justification-center is-layout-flex wp-container-core-buttons-is-layout-16018d1d wp-block-buttons-is-layout-flex\">\n<div class=\"wp-block-button\"><a class=\"wp-block-button__link wp-element-button\" href=\"https:\/\/codeanddebug.in\/course\/zero-to-hero-python-dsa\" target=\"_blank\" rel=\"noreferrer noopener\">Join our Advance DSA COURSE<\/a><\/div>\n<\/div>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<p><em>For any changes to the article, kindly email at <a href=\"mailto:code@codeanddebug.in\" target=\"_blank\" rel=\"noreferrer noopener\">code@codeanddebug.in<\/a> or contact us at <a href=\"tel:+91-9712928220\" target=\"_blank\" rel=\"noreferrer noopener\">+91-9712928220<\/a>.<\/em><\/p>\n\n\n\n<p><\/p>\n","protected":false},"excerpt":{"rendered":"<p>Given an array&nbsp;arr&nbsp;of non-negative integers and an integer&nbsp;target, the task is to count all subsets of the array whose sum is equal to the given&nbsp;target. Here&#8217;s the [Problem Link] to begin with. Examples: Input: arr[] = [5, 2, 3, 10, 6, 8], target = 10 Output: 3 Explanation: The subsets {5, 2, 3}, {2, 8},<\/p>\n","protected":false},"author":1,"featured_media":889,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"ub_ctt_via":"","footnotes":""},"categories":[3,6],"tags":[39,19],"class_list":{"0":"post-887","1":"post","2":"type-post","3":"status-publish","4":"format-standard","5":"has-post-thumbnail","7":"category-data-structures-and-algorithm","8":"category-intermediate","9":"tag-dynamic-programming-on-subsequence","10":"tag-medium"},"featured_image_src":"https:\/\/codeanddebug.in\/blog\/wp-content\/uploads\/2025\/08\/perfect-sum-problem-featured-image.png","author_info":{"display_name":"codeanddebug","author_link":"https:\/\/codeanddebug.in\/blog\/author\/codeanddebug\/"},"_links":{"self":[{"href":"https:\/\/codeanddebug.in\/blog\/wp-json\/wp\/v2\/posts\/887","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/codeanddebug.in\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/codeanddebug.in\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/codeanddebug.in\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/codeanddebug.in\/blog\/wp-json\/wp\/v2\/comments?post=887"}],"version-history":[{"count":2,"href":"https:\/\/codeanddebug.in\/blog\/wp-json\/wp\/v2\/posts\/887\/revisions"}],"predecessor-version":[{"id":891,"href":"https:\/\/codeanddebug.in\/blog\/wp-json\/wp\/v2\/posts\/887\/revisions\/891"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/codeanddebug.in\/blog\/wp-json\/wp\/v2\/media\/889"}],"wp:attachment":[{"href":"https:\/\/codeanddebug.in\/blog\/wp-json\/wp\/v2\/media?parent=887"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/codeanddebug.in\/blog\/wp-json\/wp\/v2\/categories?post=887"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/codeanddebug.in\/blog\/wp-json\/wp\/v2\/tags?post=887"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}