{"id":169,"date":"2025-05-27T11:49:19","date_gmt":"2025-05-27T06:19:19","guid":{"rendered":"https:\/\/codeanddebug.in\/blog\/?p=169"},"modified":"2025-05-27T17:40:57","modified_gmt":"2025-05-27T12:10:57","slug":"surrounded-regions-leetcode-130","status":"publish","type":"post","link":"https:\/\/codeanddebug.in\/blog\/surrounded-regions-leetcode-130\/","title":{"rendered":"Surrounded Regions | Leetcode 130 | Explained using DFS"},"content":{"rendered":"\n<p>Learn how to solve the <strong>Surrounded Regions<\/strong> problem by marking all <strong>\u2018O\u2019s<\/strong> connected to the border via DFS, then flipping the rest. Two in-place DFS solutions are explained step by step with comments.<\/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-1a4ac10e-2400-4593-b10a-09ec84edbce4\" 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\/surrounded-regions-leetcode-130\/#0-what-the-problem-asks\" style=\"\">What the Problem Asks<\/a><\/li><li style=\"\"><a href=\"https:\/\/codeanddebug.in\/blog\/surrounded-regions-leetcode-130\/#1-the-easy-idea\" style=\"\">The Easy Idea<\/a><\/li><li style=\"\"><a href=\"https:\/\/codeanddebug.in\/blog\/surrounded-regions-leetcode-130\/#2-solution-1-one-loop-over-every-border-cell\" style=\"\">Solution 1: One Loop Over Every Border Cell<\/a><ul><li style=\"\"><a href=\"https:\/\/codeanddebug.in\/blog\/surrounded-regions-leetcode-130\/#3-step-by-step-explanation\" style=\"\">Step-by-Step Explanation<\/a><\/li><\/ul><\/li><li style=\"\"><a href=\"https:\/\/codeanddebug.in\/blog\/surrounded-regions-leetcode-130\/#4-solution-2-four-separate-border-loops\" style=\"\">Solution 2: Four Separate Border Loops<\/a><ul><li style=\"\"><a href=\"https:\/\/codeanddebug.in\/blog\/surrounded-regions-leetcode-130\/#5-step-by-step-explanation\" style=\"\">Step-by-Step Explanation<\/a><\/li><\/ul><\/li><li style=\"\"><a href=\"https:\/\/codeanddebug.in\/blog\/surrounded-regions-leetcode-130\/#6-why-this-works\" style=\"\">Why This Works<\/a><\/li><li style=\"\"><a href=\"https:\/\/codeanddebug.in\/blog\/surrounded-regions-leetcode-130\/#7-time-amp-space-complexity\" style=\"\">Time &amp; Space Complexity<\/a><\/li><li style=\"\"><a href=\"https:\/\/codeanddebug.in\/blog\/surrounded-regions-leetcode-130\/#8-conclusion\" style=\"\">Conclusion<\/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 130: Surrounded Regions<\/strong><br>You have a 2D board of letters <code>'X'<\/code> and <code>'O'<\/code>. You want to flip every <code>'O'<\/code> that is completely surrounded by <code>'X'<\/code> on <strong>all four<\/strong> sides into an <code>'X'<\/code>. An <code>'O'<\/code> is <strong>not<\/strong> surrounded if it\u2019s connected (directly or indirectly) to any <code>'O'<\/code> on the border of the board.<\/p>\n\n\n\n<p>In other words:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li>Any group of <code>'O'<\/code>s entirely inside gets turned into <code>'X'<\/code>.<\/li>\n\n\n\n<li>Any group of <code>'O'<\/code>s that touches the edge of the board stays <code>'O'<\/code>.<\/li>\n<\/ul>\n<\/blockquote>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"1-the-easy-idea\">The Easy Idea<\/h2>\n\n\n\n<ol class=\"wp-block-list\">\n<li><strong>Find all \u2018O\u2019s on the border<\/strong> (top row, bottom row, left column, right column).<\/li>\n\n\n\n<li><strong>Mark every \u2018O\u2019 that connects to those border \u2018O\u2019s<\/strong> (using DFS). These are the ones we <strong>don\u2019t<\/strong> flip.<\/li>\n\n\n\n<li><strong>Flip all the other \u2018O\u2019s<\/strong>\u2014those that never got marked\u2014into \u2018X\u2019.<\/li>\n<\/ol>\n\n\n\n<p>That way, you only flip the truly enclosed regions.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"2-solution-1-one-loop-over-every-border-cell\">Solution 1: One Loop Over Every Border Cell<\/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=\"class Solution:\n    def dfs(self, r, c, visited, rows, cols, board):\n        # If out of bounds, stop\n        if r &lt; 0 or r &gt;= rows or c &lt; 0 or c &gt;= cols:\n            return\n        # If it's an 'X', stop\n        if board[r][c] == &quot;X&quot;:\n            return\n        # If already marked as border-connected, stop\n        if visited[r][c] == 1:\n            return\n        # Mark this 'O' as connected to the border\n        visited[r][c] = 1\n        # Recurse up, left, right, down\n        self.dfs(r - 1, c, visited, rows, cols, board)\n        self.dfs(r, c - 1, visited, rows, cols, board)\n        self.dfs(r, c + 1, visited, rows, cols, board)\n        self.dfs(r + 1, c, visited, rows, cols, board)\n\n    def solve(self, board: List[List[str]]) -&gt; None:\n        &quot;&quot;&quot;\n        Do not return anything, modify board in-place instead.\n        &quot;&quot;&quot;\n        rows = len(board)\n        cols = len(board[0])\n        # 0 = unvisited, 1 = connected-to-border\n        visited = [[0 for _ in range(cols)] for _ in range(rows)]\n\n        # 1. Check every cell on the border\n        for r in range(rows):\n            for c in range(cols):\n                # Is this cell on the outer border?\n                if r == 0 or c == 0 or r == rows - 1 or c == cols - 1:\n                    # If it's an 'O' and not yet visited, run DFS\n                    if board[r][c] == &quot;O&quot;:\n                        if visited[r][c] == 0:\n                            self.dfs(r, c, visited, rows, cols, board)\n\n        # 2. Flip all 'O's that were never marked\n        for r in range(rows):\n            for c in range(cols):\n                if board[r][c] == &quot;O&quot; and visited[r][c] == 0:\n                    board[r][c] = &quot;X&quot;\n\" 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\">dfs<\/span><span style=\"color: #D4D4D4\">(<\/span><span style=\"color: #9CDCFE\">self<\/span><span style=\"color: #D4D4D4\">, <\/span><span style=\"color: #9CDCFE\">r<\/span><span style=\"color: #D4D4D4\">, <\/span><span style=\"color: #9CDCFE\">c<\/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 style=\"color: #9CDCFE\">board<\/span><span style=\"color: #D4D4D4\">):<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><span style=\"color: #6A9955\"># If out of bounds, stop<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><span style=\"color: #C586C0\">if<\/span><span style=\"color: #D4D4D4\"> r &lt; <\/span><span style=\"color: #B5CEA8\">0<\/span><span style=\"color: #D4D4D4\"> <\/span><span style=\"color: #569CD6\">or<\/span><span style=\"color: #D4D4D4\"> r &gt;= rows <\/span><span style=\"color: #569CD6\">or<\/span><span style=\"color: #D4D4D4\"> c &lt; <\/span><span style=\"color: #B5CEA8\">0<\/span><span style=\"color: #D4D4D4\"> <\/span><span style=\"color: #569CD6\">or<\/span><span style=\"color: #D4D4D4\"> c &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: #6A9955\"># If it&#39;s an &#39;X&#39;, stop<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><span style=\"color: #C586C0\">if<\/span><span style=\"color: #D4D4D4\"> board[r][c] == <\/span><span style=\"color: #CE9178\">&quot;X&quot;<\/span><span style=\"color: #D4D4D4\">:<\/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: #6A9955\"># If already marked as border-connected, stop<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><span style=\"color: #C586C0\">if<\/span><span style=\"color: #D4D4D4\"> visited[r][c] == <\/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>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><span style=\"color: #6A9955\"># Mark this &#39;O&#39; as connected to the border<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        visited[r][c] = <\/span><span style=\"color: #B5CEA8\">1<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><span style=\"color: #6A9955\"># Recurse up, left, right, down<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><span style=\"color: #569CD6\">self<\/span><span style=\"color: #D4D4D4\">.dfs(r - <\/span><span style=\"color: #B5CEA8\">1<\/span><span style=\"color: #D4D4D4\">, c, visited, rows, cols, board)<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><span style=\"color: #569CD6\">self<\/span><span style=\"color: #D4D4D4\">.dfs(r, c - <\/span><span style=\"color: #B5CEA8\">1<\/span><span style=\"color: #D4D4D4\">, visited, rows, cols, board)<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><span style=\"color: #569CD6\">self<\/span><span style=\"color: #D4D4D4\">.dfs(r, c + <\/span><span style=\"color: #B5CEA8\">1<\/span><span style=\"color: #D4D4D4\">, visited, rows, cols, board)<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><span style=\"color: #569CD6\">self<\/span><span style=\"color: #D4D4D4\">.dfs(r + <\/span><span style=\"color: #B5CEA8\">1<\/span><span style=\"color: #D4D4D4\">, c, visited, rows, cols, board)<\/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\">solve<\/span><span style=\"color: #D4D4D4\">(<\/span><span style=\"color: #9CDCFE\">self<\/span><span style=\"color: #D4D4D4\">, <\/span><span style=\"color: #9CDCFE\">board<\/span><span style=\"color: #D4D4D4\">: List[List[<\/span><span style=\"color: #4EC9B0\">str<\/span><span style=\"color: #D4D4D4\">]]) -&gt; <\/span><span style=\"color: #569CD6\">None<\/span><span style=\"color: #D4D4D4\">:<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><span style=\"color: #CE9178\">&quot;&quot;&quot;<\/span><\/span>\n<span class=\"line\"><span style=\"color: #CE9178\">        Do not return anything, modify board in-place instead.<\/span><\/span>\n<span class=\"line\"><span style=\"color: #CE9178\">        &quot;&quot;&quot;<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        rows = <\/span><span style=\"color: #DCDCAA\">len<\/span><span style=\"color: #D4D4D4\">(board)<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        cols = <\/span><span style=\"color: #DCDCAA\">len<\/span><span style=\"color: #D4D4D4\">(board[<\/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: #6A9955\"># 0 = unvisited, 1 = connected-to-border<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        visited = [[<\/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\">(cols)] <\/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\">(rows)]<\/span><\/span>\n<span class=\"line\"><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><span style=\"color: #6A9955\"># 1. Check every cell on the border<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><span style=\"color: #C586C0\">for<\/span><span style=\"color: #D4D4D4\"> r <\/span><span style=\"color: #C586C0\">in<\/span><span style=\"color: #D4D4D4\"> <\/span><span style=\"color: #DCDCAA\">range<\/span><span style=\"color: #D4D4D4\">(rows):<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">            <\/span><span style=\"color: #C586C0\">for<\/span><span style=\"color: #D4D4D4\"> c <\/span><span style=\"color: #C586C0\">in<\/span><span style=\"color: #D4D4D4\"> <\/span><span style=\"color: #DCDCAA\">range<\/span><span style=\"color: #D4D4D4\">(cols):<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">                <\/span><span style=\"color: #6A9955\"># Is this cell on the outer border?<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">                <\/span><span style=\"color: #C586C0\">if<\/span><span style=\"color: #D4D4D4\"> r == <\/span><span style=\"color: #B5CEA8\">0<\/span><span style=\"color: #D4D4D4\"> <\/span><span style=\"color: #569CD6\">or<\/span><span style=\"color: #D4D4D4\"> c == <\/span><span style=\"color: #B5CEA8\">0<\/span><span style=\"color: #D4D4D4\"> <\/span><span style=\"color: #569CD6\">or<\/span><span style=\"color: #D4D4D4\"> r == rows - <\/span><span style=\"color: #B5CEA8\">1<\/span><span style=\"color: #D4D4D4\"> <\/span><span style=\"color: #569CD6\">or<\/span><span style=\"color: #D4D4D4\"> c == cols - <\/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\"># If it&#39;s an &#39;O&#39; and not yet visited, run DFS<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">                    <\/span><span style=\"color: #C586C0\">if<\/span><span style=\"color: #D4D4D4\"> board[r][c] == <\/span><span style=\"color: #CE9178\">&quot;O&quot;<\/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\"> visited[r][c] == <\/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: #569CD6\">self<\/span><span style=\"color: #D4D4D4\">.dfs(r, c, visited, rows, cols, board)<\/span><\/span>\n<span class=\"line\"><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><span style=\"color: #6A9955\"># 2. Flip all &#39;O&#39;s that were never marked<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><span style=\"color: #C586C0\">for<\/span><span style=\"color: #D4D4D4\"> r <\/span><span style=\"color: #C586C0\">in<\/span><span style=\"color: #D4D4D4\"> <\/span><span style=\"color: #DCDCAA\">range<\/span><span style=\"color: #D4D4D4\">(rows):<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">            <\/span><span style=\"color: #C586C0\">for<\/span><span style=\"color: #D4D4D4\"> c <\/span><span style=\"color: #C586C0\">in<\/span><span style=\"color: #D4D4D4\"> <\/span><span style=\"color: #DCDCAA\">range<\/span><span style=\"color: #D4D4D4\">(cols):<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">                <\/span><span style=\"color: #C586C0\">if<\/span><span style=\"color: #D4D4D4\"> board[r][c] == <\/span><span style=\"color: #CE9178\">&quot;O&quot;<\/span><span style=\"color: #D4D4D4\"> <\/span><span style=\"color: #569CD6\">and<\/span><span style=\"color: #D4D4D4\"> visited[r][c] == <\/span><span style=\"color: #B5CEA8\">0<\/span><span style=\"color: #D4D4D4\">:<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">                    board[r][c] = <\/span><span style=\"color: #CE9178\">&quot;X&quot;<\/span><\/span>\n<span class=\"line\"><\/span><\/code><\/pre><\/div>\n\n\n\n<h3 class=\"wp-block-heading\" id=\"3-step-by-step-explanation\">Step-by-Step Explanation<\/h3>\n\n\n\n<ol class=\"wp-block-list\">\n<li><strong>Build <code>visited<\/code> array<\/strong> same size as <code>board<\/code>, initialized to 0.<\/li>\n\n\n\n<li><strong>Border scan:<\/strong>\n<ul class=\"wp-block-list\">\n<li>Loop over every cell <code>(r,c)<\/code>.<\/li>\n\n\n\n<li>If it&#8217;s on the very first row, last row, first column, or last column <strong>and<\/strong> is an <code>'O'<\/code>, call <code>dfs<\/code>.<\/li>\n<\/ul>\n<\/li>\n\n\n\n<li><strong><code>dfs(r,c)<\/code><\/strong> marks the current <code>'O'<\/code> visited, then does the same for any adjacent <code>'O'<\/code>.<\/li>\n\n\n\n<li><strong>Final pass:<\/strong>\n<ul class=\"wp-block-list\">\n<li>Any <code>'O'<\/code> still unvisited must be fully surrounded\u2014flip it to <code>'X'<\/code>.<\/li>\n<\/ul>\n<\/li>\n<\/ol>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"4-solution-2-four-separate-border-loops\">Solution 2: Four Separate Border Loops<\/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=\"class Solution:\n    def dfs(self, r, c, visited, rows, cols, board):\n        if r &lt; 0 or r &gt;= rows or c &lt; 0 or c &gt;= cols:\n            return\n        if board[r][c] == &quot;X&quot;:\n            return\n        if visited[r][c] == 1:\n            return\n        visited[r][c] = 1\n        self.dfs(r - 1, c, visited, rows, cols, board)\n        self.dfs(r, c - 1, visited, rows, cols, board)\n        self.dfs(r, c + 1, visited, rows, cols, board)\n        self.dfs(r + 1, c, visited, rows, cols, board)\n\n    def solve(self, board: List[List[str]]) -&gt; None:\n        &quot;&quot;&quot;\n        Do not return anything, modify board in-place instead.\n        &quot;&quot;&quot;\n        rows = len(board)\n        cols = len(board[0])\n        visited = [[0 for _ in range(cols)] for _ in range(rows)]\n\n        # Upper Row\n        r, c = 0, 0\n        for c in range(cols):\n            if board[r][c] == &quot;O&quot;:\n                if visited[r][c] == 0:\n                    self.dfs(r, c, visited, rows, cols, board)\n\n        # Last Row\n        r, c = rows - 1, 0\n        for c in range(cols):\n            if board[r][c] == &quot;O&quot;:\n                if visited[r][c] == 0:\n                    self.dfs(r, c, visited, rows, cols, board)\n\n        # First Column\n        r, c = 0, 0\n        for r in range(rows):\n            if board[r][c] == &quot;O&quot;:\n                if visited[r][c] == 0:\n                    self.dfs(r, c, visited, rows, cols, board)\n\n        # Last Column\n        r, c = 0, cols - 1\n        for r in range(rows):\n            if board[r][c] == &quot;O&quot;:\n                if visited[r][c] == 0:\n                    self.dfs(r, c, visited, rows, cols, board)\n\n        # Flip the interior 'O's that remain unvisited\n        for r in range(rows):\n            for c in range(cols):\n                if board[r][c] == &quot;O&quot; and visited[r][c] == 0:\n                    board[r][c] = &quot;X&quot;\" 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\">dfs<\/span><span style=\"color: #D4D4D4\">(<\/span><span style=\"color: #9CDCFE\">self<\/span><span style=\"color: #D4D4D4\">, <\/span><span style=\"color: #9CDCFE\">r<\/span><span style=\"color: #D4D4D4\">, <\/span><span style=\"color: #9CDCFE\">c<\/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 style=\"color: #9CDCFE\">board<\/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\"> r &lt; <\/span><span style=\"color: #B5CEA8\">0<\/span><span style=\"color: #D4D4D4\"> <\/span><span style=\"color: #569CD6\">or<\/span><span style=\"color: #D4D4D4\"> r &gt;= rows <\/span><span style=\"color: #569CD6\">or<\/span><span style=\"color: #D4D4D4\"> c &lt; <\/span><span style=\"color: #B5CEA8\">0<\/span><span style=\"color: #D4D4D4\"> <\/span><span style=\"color: #569CD6\">or<\/span><span style=\"color: #D4D4D4\"> c &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\"> board[r][c] == <\/span><span style=\"color: #CE9178\">&quot;X&quot;<\/span><span style=\"color: #D4D4D4\">:<\/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[r][c] == <\/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>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        visited[r][c] = <\/span><span style=\"color: #B5CEA8\">1<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><span style=\"color: #569CD6\">self<\/span><span style=\"color: #D4D4D4\">.dfs(r - <\/span><span style=\"color: #B5CEA8\">1<\/span><span style=\"color: #D4D4D4\">, c, visited, rows, cols, board)<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><span style=\"color: #569CD6\">self<\/span><span style=\"color: #D4D4D4\">.dfs(r, c - <\/span><span style=\"color: #B5CEA8\">1<\/span><span style=\"color: #D4D4D4\">, visited, rows, cols, board)<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><span style=\"color: #569CD6\">self<\/span><span style=\"color: #D4D4D4\">.dfs(r, c + <\/span><span style=\"color: #B5CEA8\">1<\/span><span style=\"color: #D4D4D4\">, visited, rows, cols, board)<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><span style=\"color: #569CD6\">self<\/span><span style=\"color: #D4D4D4\">.dfs(r + <\/span><span style=\"color: #B5CEA8\">1<\/span><span style=\"color: #D4D4D4\">, c, visited, rows, cols, board)<\/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\">solve<\/span><span style=\"color: #D4D4D4\">(<\/span><span style=\"color: #9CDCFE\">self<\/span><span style=\"color: #D4D4D4\">, <\/span><span style=\"color: #9CDCFE\">board<\/span><span style=\"color: #D4D4D4\">: List[List[<\/span><span style=\"color: #4EC9B0\">str<\/span><span style=\"color: #D4D4D4\">]]) -&gt; <\/span><span style=\"color: #569CD6\">None<\/span><span style=\"color: #D4D4D4\">:<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><span style=\"color: #CE9178\">&quot;&quot;&quot;<\/span><\/span>\n<span class=\"line\"><span style=\"color: #CE9178\">        Do not return anything, modify board in-place instead.<\/span><\/span>\n<span class=\"line\"><span style=\"color: #CE9178\">        &quot;&quot;&quot;<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        rows = <\/span><span style=\"color: #DCDCAA\">len<\/span><span style=\"color: #D4D4D4\">(board)<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        cols = <\/span><span style=\"color: #DCDCAA\">len<\/span><span style=\"color: #D4D4D4\">(board[<\/span><span style=\"color: #B5CEA8\">0<\/span><span style=\"color: #D4D4D4\">])<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        visited = [[<\/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\">(cols)] <\/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\">(rows)]<\/span><\/span>\n<span class=\"line\"><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><span style=\"color: #6A9955\"># Upper Row<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        r, c = <\/span><span style=\"color: #B5CEA8\">0<\/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: #C586C0\">for<\/span><span style=\"color: #D4D4D4\"> c <\/span><span style=\"color: #C586C0\">in<\/span><span style=\"color: #D4D4D4\"> <\/span><span style=\"color: #DCDCAA\">range<\/span><span style=\"color: #D4D4D4\">(cols):<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">            <\/span><span style=\"color: #C586C0\">if<\/span><span style=\"color: #D4D4D4\"> board[r][c] == <\/span><span style=\"color: #CE9178\">&quot;O&quot;<\/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\"> visited[r][c] == <\/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: #569CD6\">self<\/span><span style=\"color: #D4D4D4\">.dfs(r, c, visited, rows, cols, board)<\/span><\/span>\n<span class=\"line\"><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><span style=\"color: #6A9955\"># Last Row<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        r, c = rows - <\/span><span style=\"color: #B5CEA8\">1<\/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: #C586C0\">for<\/span><span style=\"color: #D4D4D4\"> c <\/span><span style=\"color: #C586C0\">in<\/span><span style=\"color: #D4D4D4\"> <\/span><span style=\"color: #DCDCAA\">range<\/span><span style=\"color: #D4D4D4\">(cols):<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">            <\/span><span style=\"color: #C586C0\">if<\/span><span style=\"color: #D4D4D4\"> board[r][c] == <\/span><span style=\"color: #CE9178\">&quot;O&quot;<\/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\"> visited[r][c] == <\/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: #569CD6\">self<\/span><span style=\"color: #D4D4D4\">.dfs(r, c, visited, rows, cols, board)<\/span><\/span>\n<span class=\"line\"><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><span style=\"color: #6A9955\"># First Column<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        r, c = <\/span><span style=\"color: #B5CEA8\">0<\/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: #C586C0\">for<\/span><span style=\"color: #D4D4D4\"> r <\/span><span style=\"color: #C586C0\">in<\/span><span style=\"color: #D4D4D4\"> <\/span><span style=\"color: #DCDCAA\">range<\/span><span style=\"color: #D4D4D4\">(rows):<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">            <\/span><span style=\"color: #C586C0\">if<\/span><span style=\"color: #D4D4D4\"> board[r][c] == <\/span><span style=\"color: #CE9178\">&quot;O&quot;<\/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\"> visited[r][c] == <\/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: #569CD6\">self<\/span><span style=\"color: #D4D4D4\">.dfs(r, c, visited, rows, cols, board)<\/span><\/span>\n<span class=\"line\"><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><span style=\"color: #6A9955\"># Last Column<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        r, c = <\/span><span style=\"color: #B5CEA8\">0<\/span><span style=\"color: #D4D4D4\">, cols - <\/span><span style=\"color: #B5CEA8\">1<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><span style=\"color: #C586C0\">for<\/span><span style=\"color: #D4D4D4\"> r <\/span><span style=\"color: #C586C0\">in<\/span><span style=\"color: #D4D4D4\"> <\/span><span style=\"color: #DCDCAA\">range<\/span><span style=\"color: #D4D4D4\">(rows):<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">            <\/span><span style=\"color: #C586C0\">if<\/span><span style=\"color: #D4D4D4\"> board[r][c] == <\/span><span style=\"color: #CE9178\">&quot;O&quot;<\/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\"> visited[r][c] == <\/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: #569CD6\">self<\/span><span style=\"color: #D4D4D4\">.dfs(r, c, visited, rows, cols, board)<\/span><\/span>\n<span class=\"line\"><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><span style=\"color: #6A9955\"># Flip the interior &#39;O&#39;s that remain unvisited<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">        <\/span><span style=\"color: #C586C0\">for<\/span><span style=\"color: #D4D4D4\"> r <\/span><span style=\"color: #C586C0\">in<\/span><span style=\"color: #D4D4D4\"> <\/span><span style=\"color: #DCDCAA\">range<\/span><span style=\"color: #D4D4D4\">(rows):<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">            <\/span><span style=\"color: #C586C0\">for<\/span><span style=\"color: #D4D4D4\"> c <\/span><span style=\"color: #C586C0\">in<\/span><span style=\"color: #D4D4D4\"> <\/span><span style=\"color: #DCDCAA\">range<\/span><span style=\"color: #D4D4D4\">(cols):<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">                <\/span><span style=\"color: #C586C0\">if<\/span><span style=\"color: #D4D4D4\"> board[r][c] == <\/span><span style=\"color: #CE9178\">&quot;O&quot;<\/span><span style=\"color: #D4D4D4\"> <\/span><span style=\"color: #569CD6\">and<\/span><span style=\"color: #D4D4D4\"> visited[r][c] == <\/span><span style=\"color: #B5CEA8\">0<\/span><span style=\"color: #D4D4D4\">:<\/span><\/span>\n<span class=\"line\"><span style=\"color: #D4D4D4\">                    board[r][c] = <\/span><span style=\"color: #CE9178\">&quot;X&quot;<\/span><\/span><\/code><\/pre><\/div>\n\n\n\n<h3 class=\"wp-block-heading\" id=\"5-step-by-step-explanation\">Step-by-Step Explanation<\/h3>\n\n\n\n<ol class=\"wp-block-list\">\n<li><strong>Initialize<\/strong> <code>visited<\/code> to all zeros.<\/li>\n\n\n\n<li><strong>Four border sweeps:<\/strong>\n<ul class=\"wp-block-list\">\n<li>Go across the top row, bottom row, left column, then right column.<\/li>\n\n\n\n<li>For each border cell that\u2019s an <code>'O'<\/code> and unvisited, call <code>dfs<\/code>.<\/li>\n<\/ul>\n<\/li>\n\n\n\n<li><strong><code>dfs<\/code><\/strong> marks all <code>'O'<\/code>s connected to that border cell.<\/li>\n\n\n\n<li><strong>Final sweep:<\/strong>\n<ul class=\"wp-block-list\">\n<li>Any <code>'O'<\/code> still unvisited lies in a fully surrounded region\u2014flip it to <code>'X'<\/code>.<\/li>\n<\/ul>\n<\/li>\n<\/ol>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"6-why-this-works\">Why This Works<\/h2>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>Any <code>'O'<\/code> that can\u2019t reach the border<\/strong> (via up\/down\/left\/right moves) is surrounded.<\/li>\n\n\n\n<li><strong>DFS<\/strong> from border cells marks exactly those <code>'O'<\/code>s that should stay.<\/li>\n\n\n\n<li><strong>All others<\/strong> get flipped in a single final pass.<\/li>\n<\/ul>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"7-time-amp-space-complexity\">Time &amp; Space Complexity<\/h2>\n\n\n\n<ol class=\"wp-block-list\">\n<li><strong>Time Complexity:<\/strong> O(m\u00b7n.4)\n<ul class=\"wp-block-list\">\n<li>Each DFS visits each cell at most once with looking into every direction.<\/li>\n<\/ul>\n<\/li>\n\n\n\n<li><strong>Space Complexity:<\/strong> O(m\u00b7n)\n<ul class=\"wp-block-list\">\n<li>The <code>visited<\/code> matrix and call stack (in worst case) can grow to the whole board.<\/li>\n<\/ul>\n<\/li>\n<\/ol>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"8-conclusion\">Conclusion<\/h2>\n\n\n\n<p>By first marking all <code>'O'<\/code> cells connected to the border (the only ones that shouldn\u2019t be flipped), we can safely flip every other <code>'O'<\/code> to <code>'X'<\/code>. Both solutions run in linear time and update the board in-place. Choose whichever border-traversal style you find more readable!<\/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\n\n\n<p><\/p>\n","protected":false},"excerpt":{"rendered":"<p>Learn how to solve the Surrounded Regions problem by marking all \u2018O\u2019s connected to the border via DFS, then flipping the rest. Two in-place DFS solutions are explained step by step with comments. What the Problem Asks The Easy Idea That way, you only flip the truly enclosed regions. Solution 1: One Loop Over Every<\/p>\n","protected":false},"author":1,"featured_media":172,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[3,5],"tags":[17],"class_list":{"0":"post-169","1":"post","2":"type-post","3":"status-publish","4":"format-standard","5":"has-post-thumbnail","7":"category-data-structures-and-algorithm","8":"category-expert","9":"tag-graph"},"featured_image_src":"https:\/\/codeanddebug.in\/blog\/wp-content\/uploads\/2025\/05\/surrounded-regions-leetcode-130-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\/169","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=169"}],"version-history":[{"count":3,"href":"https:\/\/codeanddebug.in\/blog\/wp-json\/wp\/v2\/posts\/169\/revisions"}],"predecessor-version":[{"id":179,"href":"https:\/\/codeanddebug.in\/blog\/wp-json\/wp\/v2\/posts\/169\/revisions\/179"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/codeanddebug.in\/blog\/wp-json\/wp\/v2\/media\/172"}],"wp:attachment":[{"href":"https:\/\/codeanddebug.in\/blog\/wp-json\/wp\/v2\/media?parent=169"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/codeanddebug.in\/blog\/wp-json\/wp\/v2\/categories?post=169"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/codeanddebug.in\/blog\/wp-json\/wp\/v2\/tags?post=169"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}