{"id":144,"date":"2025-05-24T18:34:58","date_gmt":"2025-05-24T13:04:58","guid":{"rendered":"https:\/\/codeanddebug.in\/blog\/?p=144"},"modified":"2025-05-24T18:37:34","modified_gmt":"2025-05-24T13:07:34","slug":"flood-fill-leetcode-733","status":"publish","type":"post","link":"https:\/\/codeanddebug.in\/blog\/flood-fill-leetcode-733\/","title":{"rendered":"Flood Fill | Leetcode 733 | DFS and BFS Approach"},"content":{"rendered":"\n<p>Discover how to implement the classic <strong>\u201cFlood Fill\u201d algorithm<\/strong> on a 2D image grid using both <strong>Depth-First Search (DFS)<\/strong> and <strong>Breadth-First Search (BFS)<\/strong>. Step-by-step code, explanations, and complexity analysis included for your LeetCode prep.<\/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-9f42e1ae-d21a-4ca7-a74c-4b4dc00d97bb\" data-linktodivider=\"false\" data-showtext=\"show\" data-hidetext=\"hide\" data-scrolltype=\"auto\" data-enablesmoothscroll=\"true\" data-initiallyhideonmobile=\"false\" 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\">Content:<\/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\/flood-fill-leetcode-733\/#0-what-the-problem-asks\" style=\"\">What the Problem Asks<\/a><\/li><li style=\"\"><a href=\"https:\/\/codeanddebug.in\/blog\/flood-fill-leetcode-733\/#1-common-setup\" style=\"\">Common Setup<\/a><\/li><li style=\"\"><a href=\"https:\/\/codeanddebug.in\/blog\/flood-fill-leetcode-733\/#2-dfs-solution-recursion\" style=\"\">DFS Solution (Recursion)<\/a><\/li><li style=\"\"><a href=\"https:\/\/codeanddebug.in\/blog\/flood-fill-leetcode-733\/#3-how-dfs-works\" style=\"\">How DFS works?<\/a><\/li><li style=\"\"><a href=\"https:\/\/codeanddebug.in\/blog\/flood-fill-leetcode-733\/#4-bfs-solution-using-a-deque\" style=\"\">BFS Solution (Using a deque)<\/a><\/li><li style=\"\"><a href=\"https:\/\/codeanddebug.in\/blog\/flood-fill-leetcode-733\/#5-how-bfs-works\" style=\"\">How BFS works?<\/a><\/li><li style=\"\"><a href=\"https:\/\/codeanddebug.in\/blog\/flood-fill-leetcode-733\/#6-time-amp-space-complexity\" style=\"\">Time &amp; Space Complexity<\/a><\/li><li style=\"\"><a href=\"https:\/\/codeanddebug.in\/blog\/flood-fill-leetcode-733\/#7-which-to-use\" style=\"\">Which to Use?<\/a><\/li><\/ul>\n\t\t\t<\/div>\n\t\t<\/div><\/div>\n\n\n<h2 class=\"wp-block-heading\" id=\"0-what-the-problem-asks\">What the Problem Asks<\/h2>\n\n\n\n<blockquote class=\"wp-block-quote is-layout-flow wp-block-quote-is-layout-flow\">\n<p><strong>LeetCode 733: Flood Fill<\/strong><br>You\u2019re given a 2D grid <code>image<\/code> of colors (integers), and a starting pixel <code>(sr, sc)<\/code>.<br>Every step, you can change the color of the current pixel\u2019s <strong>4-directional<\/strong> neighbors if they match the <strong>initial<\/strong> color at <code>(sr, sc)<\/code>.<br>Return the image after <strong>filling<\/strong> (changing) all reachable pixels to the new <code>color<\/code>.<\/p>\n<\/blockquote>\n\n\n\n<p>In plain English: starting from <code>(sr, sc)<\/code>, repaint every connected region of the same original color to a new color, either by diving deep (DFS) or spreading out level by level (BFS).<\/p>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"1-common-setup\">Common Setup<\/h2>\n\n\n\n<p>In <strong>both<\/strong> solutions we:<\/p>\n\n\n\n<ol class=\"wp-block-list\">\n<li><strong>Check<\/strong> if the start pixel already has the target <code>color<\/code>. If so, we can return <code>image<\/code> immediately.<\/li>\n\n\n\n<li><strong>Make a copy<\/strong> of <code>image<\/code> to avoid mutating the input (optional, but shown here).<\/li>\n\n\n\n<li><strong>Record<\/strong> the <code>initial_color<\/code> at <code>(sr, sc)<\/code>.<\/li>\n\n\n\n<li><strong>Drive<\/strong> our search (DFS or BFS) from <code>(sr, sc)<\/code>, changing each visited pixel to <code>color<\/code> and visiting only neighbors that still have <code>initial_color<\/code>.<\/li>\n<\/ol>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"2-dfs-solution-recursion\">DFS Solution (Recursion)<\/h2>\n\n\n\n<div class=\"wp-block-kevinbatdorf-code-block-pro\" 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;line-height:1.25rem;--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=\"from copy import deepcopy\n\n\nclass Solution:\n    def dfs(self, i, j, new_color, initial_color, visited, rows, cols):\n        if i &lt; 0 or i &gt;= rows or j &lt; 0 or j &gt;= cols:\n            return\n        if visited[i][j] != initial_color:\n            return\n        if visited[i][j] == initial_color:\n            visited[i][j] = new_color\n            \n        self.dfs(i + 1, j, new_color, initial_color, visited, rows, cols)\n        self.dfs(i, j + 1, new_color, initial_color, visited, rows, cols)\n        self.dfs(i - 1, j, new_color, initial_color, visited, rows, cols)\n        self.dfs(i, j - 1, new_color, initial_color, visited, rows, cols)\n\n    def floodFill(\n        self, image: List[List[int]], sr: int, sc: int, color: int\n    ) -&gt; List[List[int]]:\n        if image[sr][sc] == color:\n            return image\n            \n        visited = deepcopy(image)\n        rows = len(visited)\n        cols = len(visited[0])\n        initial_color = visited[sr][sc]\n        \n        self.dfs(sr, sc, color, initial_color, visited, rows, cols)\n        \n        return visited\" 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: #C586C0\">from<\/span><span style=\"color: #D4D4D4\"> copy <\/span><span style=\"color: #C586C0\">import<\/span><span style=\"color: #D4D4D4\"> deepcopy<\/span><\/span>\n<span class=\"line\"><\/span>\n<span class=\"line\"><\/span>\n<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\">dfs<\/span><span style=\"color: #D4D4D4\">(<\/span><span style=\"color: #9CDCFE\">self<\/span><span style=\"color: #D4D4D4\">, <\/span><span style=\"color: #9CDCFE\">i<\/span><span style=\"color: #D4D4D4\">, <\/span><span style=\"color: #9CDCFE\">j<\/span><span style=\"color: #D4D4D4\">, <\/span><span style=\"color: #9CDCFE\">new_color<\/span><span style=\"color: #D4D4D4\">, <\/span><span style=\"color: #9CDCFE\">initial_color<\/span><span style=\"color: #D4D4D4\">, <\/span><span style=\"color: #9CDCFE\">visited<\/span><span style=\"color: #D4D4D4\">, <\/span><span style=\"color: #9CDCFE\">rows<\/span><span style=\"color: #D4D4D4\">, <\/span><span style=\"color: #9CDCFE\">cols<\/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\"> i &lt; <\/span><span style=\"color: #B5CEA8\">0<\/span><span style=\"color: #D4D4D4\"> <\/span><span style=\"color: #569CD6\">or<\/span><span style=\"color: #D4D4D4\"> i &gt;= rows <\/span><span style=\"color: #569CD6\">or<\/span><span style=\"color: #D4D4D4\"> j &lt; <\/span><span style=\"color: #B5CEA8\">0<\/span><span style=\"color: #D4D4D4\"> <\/span><span style=\"color: #569CD6\">or<\/span><span style=\"color: #D4D4D4\"> j &gt;= cols:<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">            <\/span><span style=\"color: #C586C0\">return<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><span style=\"color: #C586C0\">if<\/span><span style=\"color: #D4D4D4\"> visited[i][j] != initial_color:<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">            <\/span><span style=\"color: #C586C0\">return<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><span style=\"color: #C586C0\">if<\/span><span style=\"color: #D4D4D4\"> visited[i][j] == initial_color:<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">            visited[i][j] = new_color<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">            <\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><span style=\"color: #569CD6\">self<\/span><span style=\"color: #D4D4D4\">.dfs(i + <\/span><span style=\"color: #B5CEA8\">1<\/span><span style=\"color: #D4D4D4\">, j, new_color, initial_color, visited, rows, cols)<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><span style=\"color: #569CD6\">self<\/span><span style=\"color: #D4D4D4\">.dfs(i, j + <\/span><span style=\"color: #B5CEA8\">1<\/span><span style=\"color: #D4D4D4\">, new_color, initial_color, visited, rows, cols)<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><span style=\"color: #569CD6\">self<\/span><span style=\"color: #D4D4D4\">.dfs(i - <\/span><span style=\"color: #B5CEA8\">1<\/span><span style=\"color: #D4D4D4\">, j, new_color, initial_color, visited, rows, cols)<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><span style=\"color: #569CD6\">self<\/span><span style=\"color: #D4D4D4\">.dfs(i, j - <\/span><span style=\"color: #B5CEA8\">1<\/span><span style=\"color: #D4D4D4\">, new_color, initial_color, visited, rows, cols)<\/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\">floodFill<\/span><span style=\"color: #D4D4D4\">(<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><span style=\"color: #9CDCFE\">self<\/span><span style=\"color: #D4D4D4\">, <\/span><span style=\"color: #9CDCFE\">image<\/span><span style=\"color: #D4D4D4\">: List[List[<\/span><span style=\"color: #4EC9B0\">int<\/span><span style=\"color: #D4D4D4\">]], <\/span><span style=\"color: #9CDCFE\">sr<\/span><span style=\"color: #D4D4D4\">: <\/span><span style=\"color: #4EC9B0\">int<\/span><span style=\"color: #D4D4D4\">, <\/span><span style=\"color: #9CDCFE\">sc<\/span><span style=\"color: #D4D4D4\">: <\/span><span style=\"color: #4EC9B0\">int<\/span><span style=\"color: #D4D4D4\">, <\/span><span style=\"color: #9CDCFE\">color<\/span><span style=\"color: #D4D4D4\">: <\/span><span style=\"color: #4EC9B0\">int<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">    ) -&gt; List[List[<\/span><span style=\"color: #4EC9B0\">int<\/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\"> image[sr][sc] == color:<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">            <\/span><span style=\"color: #C586C0\">return<\/span><span style=\"color: #D4D4D4\"> image<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">            <\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        visited = deepcopy(image)<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        rows = <\/span><span style=\"color: #DCDCAA\">len<\/span><span style=\"color: #D4D4D4\">(visited)<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        cols = <\/span><span style=\"color: #DCDCAA\">len<\/span><span style=\"color: #D4D4D4\">(visited[<\/span><span style=\"color: #B5CEA8\">0<\/span><span style=\"color: #D4D4D4\">])<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        initial_color = visited[sr][sc]<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><span style=\"color: #569CD6\">self<\/span><span style=\"color: #D4D4D4\">.dfs(sr, sc, color, initial_color, visited, rows, cols)<\/span><\/span>\n<span class=\"line\"><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\"> visited<\/span><\/span><\/code><\/pre><\/div>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"3-how-dfs-works\">How DFS works?<\/h2>\n\n\n\n<ol class=\"wp-block-list\">\n<li><strong><code>dfs(i, j)<\/code><\/strong>:\n<ul class=\"wp-block-list\">\n<li>Paints <code>(i, j)<\/code> to <code>color<\/code>.<\/li>\n\n\n\n<li>Recursively calls itself on any neighbor that is still <code>initial<\/code> color.<\/li>\n<\/ul>\n<\/li>\n\n\n\n<li><strong>Call<\/strong> <code>dfs(sr, sc)<\/code> once to flood the entire connected region.<\/li>\n<\/ol>\n\n\n\n<p>This is a classic <strong>recursive DFS<\/strong>\u2014you go as deep as possible from each pixel, then backtrack.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"4-bfs-solution-using-a-deque\">BFS Solution (Using a deque)<\/h2>\n\n\n\n<div class=\"wp-block-kevinbatdorf-code-block-pro\" 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;line-height:1.25rem;--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=\"from copy import deepcopy\nfrom collections import deque\n\nclass Solution:\n    def floodFill(\n        self, image: List[List[int]], sr: int, sc: int, color: int\n    ) -&gt; List[List[int]]:\n        if image[sr][sc] == color:\n            return image\n            \n        visited = deepcopy(image)\n        rows = len(visited)\n        cols = len(visited[0])\n        initial_color = visited[sr][sc]\n        \n        queue = deque()\n        queue.append((sr, sc))\n        \n        while len(queue) != 0:\n            i, j = queue.popleft()\n            visited[i][j] = color\n            for x, y in [(-1, 0), (0, -1), (1, 0), (0, 1)]:\n                new_i = i + x\n                new_j = j + y\n                if new_i &lt; 0 or new_i &gt;= rows or new_j &lt; 0 or new_j &gt;= cols:\n                    continue\n                if visited[new_i][new_j] != initial_color:\n                    continue\n                queue.append((new_i, new_j))\n        return visited\" 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: #C586C0\">from<\/span><span style=\"color: #D4D4D4\"> copy <\/span><span style=\"color: #C586C0\">import<\/span><span style=\"color: #D4D4D4\"> deepcopy<\/span><\/span>\n<span class=\"line\"><span style=\"color: #C586C0\">from<\/span><span style=\"color: #D4D4D4\"> collections <\/span><span style=\"color: #C586C0\">import<\/span><span style=\"color: #D4D4D4\"> deque<\/span><\/span>\n<span class=\"line\"><\/span>\n<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\">floodFill<\/span><span style=\"color: #D4D4D4\">(<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><span style=\"color: #9CDCFE\">self<\/span><span style=\"color: #D4D4D4\">, <\/span><span style=\"color: #9CDCFE\">image<\/span><span style=\"color: #D4D4D4\">: List[List[<\/span><span style=\"color: #4EC9B0\">int<\/span><span style=\"color: #D4D4D4\">]], <\/span><span style=\"color: #9CDCFE\">sr<\/span><span style=\"color: #D4D4D4\">: <\/span><span style=\"color: #4EC9B0\">int<\/span><span style=\"color: #D4D4D4\">, <\/span><span style=\"color: #9CDCFE\">sc<\/span><span style=\"color: #D4D4D4\">: <\/span><span style=\"color: #4EC9B0\">int<\/span><span style=\"color: #D4D4D4\">, <\/span><span style=\"color: #9CDCFE\">color<\/span><span style=\"color: #D4D4D4\">: <\/span><span style=\"color: #4EC9B0\">int<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">    ) -&gt; List[List[<\/span><span style=\"color: #4EC9B0\">int<\/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\"> image[sr][sc] == color:<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">            <\/span><span style=\"color: #C586C0\">return<\/span><span style=\"color: #D4D4D4\"> image<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">            <\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        visited = deepcopy(image)<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        rows = <\/span><span style=\"color: #DCDCAA\">len<\/span><span style=\"color: #D4D4D4\">(visited)<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        cols = <\/span><span style=\"color: #DCDCAA\">len<\/span><span style=\"color: #D4D4D4\">(visited[<\/span><span style=\"color: #B5CEA8\">0<\/span><span style=\"color: #D4D4D4\">])<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        initial_color = visited[sr][sc]<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        queue = deque()<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        queue.append((sr, sc))<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><span style=\"color: #C586C0\">while<\/span><span style=\"color: #D4D4D4\"> <\/span><span style=\"color: #DCDCAA\">len<\/span><span style=\"color: #D4D4D4\">(queue) != <\/span><span style=\"color: #B5CEA8\">0<\/span><span style=\"color: #D4D4D4\">:<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">            i, j = queue.popleft()<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">            visited[i][j] = color<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">            <\/span><span style=\"color: #C586C0\">for<\/span><span style=\"color: #D4D4D4\"> x, y <\/span><span style=\"color: #C586C0\">in<\/span><span style=\"color: #D4D4D4\"> [(-<\/span><span style=\"color: #B5CEA8\">1<\/span><span style=\"color: #D4D4D4\">, <\/span><span style=\"color: #B5CEA8\">0<\/span><span style=\"color: #D4D4D4\">), (<\/span><span style=\"color: #B5CEA8\">0<\/span><span style=\"color: #D4D4D4\">, -<\/span><span style=\"color: #B5CEA8\">1<\/span><span style=\"color: #D4D4D4\">), (<\/span><span style=\"color: #B5CEA8\">1<\/span><span style=\"color: #D4D4D4\">, <\/span><span style=\"color: #B5CEA8\">0<\/span><span style=\"color: #D4D4D4\">), (<\/span><span style=\"color: #B5CEA8\">0<\/span><span style=\"color: #D4D4D4\">, <\/span><span style=\"color: #B5CEA8\">1<\/span><span style=\"color: #D4D4D4\">)]:<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">                new_i = i + x<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">                new_j = j + y<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">                <\/span><span style=\"color: #C586C0\">if<\/span><span style=\"color: #D4D4D4\"> new_i &lt; <\/span><span style=\"color: #B5CEA8\">0<\/span><span style=\"color: #D4D4D4\"> <\/span><span style=\"color: #569CD6\">or<\/span><span style=\"color: #D4D4D4\"> new_i &gt;= rows <\/span><span style=\"color: #569CD6\">or<\/span><span style=\"color: #D4D4D4\"> new_j &lt; <\/span><span style=\"color: #B5CEA8\">0<\/span><span style=\"color: #D4D4D4\"> <\/span><span style=\"color: #569CD6\">or<\/span><span style=\"color: #D4D4D4\"> new_j &gt;= cols:<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">                    <\/span><span style=\"color: #C586C0\">continue<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">                <\/span><span style=\"color: #C586C0\">if<\/span><span style=\"color: #D4D4D4\"> visited[new_i][new_j] != initial_color:<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">                    <\/span><span style=\"color: #C586C0\">continue<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">                queue.append((new_i, new_j))<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><span style=\"color: #C586C0\">return<\/span><span style=\"color: #D4D4D4\"> visited<\/span><\/span><\/code><\/pre><\/div>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"5-how-bfs-works\">How BFS works?<\/h2>\n\n\n\n<p><strong>Initialize<\/strong> the queue with the start pixel and paint it.<\/p>\n\n\n\n<p><strong>While queue isn\u2019t empty:<\/strong><\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li>Pop <code>(i, j)<\/code>.<\/li>\n\n\n\n<li>For each 4-directional neighbor <code>(ni, nj)<\/code>, if it\u2019s still <code>initial<\/code> color, paint it and enqueue it.<\/li>\n<\/ul>\n\n\n\n<p>You spread out in <strong>\u201cwaves\u201d<\/strong>, ensuring every pixel at distance 1, then distance 2, etc., gets recolored.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"6-time-amp-space-complexity\">Time &amp; Space Complexity<\/h2>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>Time Complexity:<\/strong> O(m \u00d7 n) \u2014 each pixel is visited at most once.<\/li>\n\n\n\n<li><strong>Space Complexity:<\/strong>\n<ul class=\"wp-block-list\">\n<li><strong>DFS recursion:<\/strong> O(m \u00d7 n) in the worst case (skewed region) for the call stack.<\/li>\n\n\n\n<li><strong>BFS queue:<\/strong> O(m \u00d7 n) in the worst case if the region spans the whole grid.<\/li>\n<\/ul>\n<\/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=\"7-which-to-use\">Which to Use?<\/h2>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>DFS<\/strong> is concise and easy with recursion, but beware of Python\u2019s recursion limit on large grids.<\/li>\n\n\n\n<li><strong>BFS<\/strong> uses an explicit queue and runs in <strong>iterative<\/strong> fashion, safer when recursion depth might blow up.<\/li>\n<\/ul>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<p><strong>Wrapping Up:<\/strong><br>Flood fill is just a graph-search problem on a grid. Whether you dive deep first (DFS) or expand in waves (BFS), the core idea is the same: visit every connected pixel of the original color and repaint it. Pick the style that best matches your comfort and constraints\u2014both work beautifully!<\/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:\/\/www.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\">code@codeanddebug.in<\/a> or contact us at <a href=\"tel:+91-9712928220\">+91-9712928220<\/a>.<\/em><\/p>\n","protected":false},"excerpt":{"rendered":"<p>Discover how to implement the classic \u201cFlood Fill\u201d algorithm on a 2D image grid using both Depth-First Search (DFS) and Breadth-First Search (BFS). Step-by-step code, explanations, and complexity analysis included for your LeetCode prep. What the Problem Asks LeetCode 733: Flood FillYou\u2019re given a 2D grid image of colors (integers), and a starting pixel (sr,<\/p>\n","protected":false},"author":1,"featured_media":148,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"ub_ctt_via":"","footnotes":""},"categories":[3,6],"tags":[17],"class_list":{"0":"post-144","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-graph"},"featured_image_src":"https:\/\/codeanddebug.in\/blog\/wp-content\/uploads\/2025\/05\/flood-fill-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\/144","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=144"}],"version-history":[{"count":2,"href":"https:\/\/codeanddebug.in\/blog\/wp-json\/wp\/v2\/posts\/144\/revisions"}],"predecessor-version":[{"id":151,"href":"https:\/\/codeanddebug.in\/blog\/wp-json\/wp\/v2\/posts\/144\/revisions\/151"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/codeanddebug.in\/blog\/wp-json\/wp\/v2\/media\/148"}],"wp:attachment":[{"href":"https:\/\/codeanddebug.in\/blog\/wp-json\/wp\/v2\/media?parent=144"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/codeanddebug.in\/blog\/wp-json\/wp\/v2\/categories?post=144"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/codeanddebug.in\/blog\/wp-json\/wp\/v2\/tags?post=144"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}