From fdb2d87fb1414eab8f0f53e4e6c64b9e62a1d761 Mon Sep 17 00:00:00 2001
From: Evan Ferrao <evanferrao@gmail.com>
Date: Sat, 14 Feb 2026 19:41:40 +0530
Subject: [PATCH] surrounded regions

---
 Graphs/SurroundedRegions.java | 141 ++++++++++++++++++++++++++++++++++
 1 file changed, 141 insertions(+)
 create mode 100644 Graphs/SurroundedRegions.java

diff --git a/Graphs/SurroundedRegions.java b/Graphs/SurroundedRegions.java
new file mode 100644
index 0000000..d11e7c0
--- /dev/null
+++ b/Graphs/SurroundedRegions.java
@@ -0,0 +1,141 @@
+// https://leetcode.com/problems/surrounded-regions/
+
+import java.util.*;
+import java.io.*;
+
+class Main {
+
+    static class Node{
+        int row;
+        int col;
+
+        Node(int row, int col){
+            this.row = row;
+            this.col = col;
+        }
+    }
+
+    // DFS
+    public static void dfs(Node node, char board[][], boolean doNotTouch[][]){
+
+        int m = board.length;
+        int n = board[0].length;
+
+        int r = node.row;
+        int c = node.col;
+
+        doNotTouch[r][c] = true;
+
+        int dr[] = {-1, 0, 1, 0};
+        int dc[] = {0, 1, 0, -1};
+
+        for(int k=0;k<4;k++){
+            int nr = r + dr[k];
+            int nc = c + dc[k];
+
+            if(nr>=0 && nc>=0 && nr<m && nc<n &&
+               !doNotTouch[nr][nc] && board[nr][nc]=='O'){
+                dfs(new Node(nr, nc), board, doNotTouch);
+            }
+        }
+    }
+
+
+    // BFS
+    public static void bfs(Node start, char board[][], boolean doNotTouch[][]){
+
+        int m = board.length;
+        int n = board[0].length;
+
+        Queue<Node> q = new LinkedList<>();
+        q.add(start);
+        doNotTouch[start.row][start.col] = true;
+
+        int dr[] = {-1, 0, 1, 0};
+        int dc[] = {0, 1, 0, -1};
+
+        while(!q.isEmpty()){
+            Node cur = q.poll();
+
+            for(int k=0;k<4;k++){
+                int nr = cur.row + dr[k];
+                int nc = cur.col + dc[k];
+
+                if(nr>=0 && nc>=0 && nr<m && nc<n &&
+                   !doNotTouch[nr][nc] && board[nr][nc]=='O'){
+                    doNotTouch[nr][nc] = true;
+                    q.add(new Node(nr, nc));
+                }
+            }
+        }
+    }
+
+
+    public static void solve(char board[][]){
+
+        int m = board.length;
+        int n = board[0].length;
+
+        boolean doNotTouch[][] = new boolean[m][n];
+
+        // borders
+        for(int i=0;i<m;i++){
+            if(board[i][0]=='O'){
+                dfs(new Node(i,0), board, doNotTouch);
+                // bfs(new Node(i,0), board, doNotTouch);
+            }
+            if(board[i][n-1]=='O'){
+                dfs(new Node(i,n-1), board, doNotTouch);
+                // bfs(new Node(i,n-1), board, doNotTouch);
+            }
+        }
+
+        for(int j=0;j<n;j++){
+            if(board[0][j]=='O'){
+                dfs(new Node(0,j), board, doNotTouch);
+                // bfs(new Node(0,j), board, doNotTouch);
+            }
+            if(board[m-1][j]=='O'){
+                dfs(new Node(m-1,j), board, doNotTouch);
+                // bfs(new Node(m-1,j), board, doNotTouch);
+            }
+        }
+
+        char finalAns[][] = new char[m][n];
+
+        for(int i=0;i<m;i++){
+            for(int j=0;j<n;j++){
+                if(doNotTouch[i][j]) finalAns[i][j] = board[i][j];
+                else finalAns[i][j] = 'X';
+            }
+        }
+
+        // copy back
+        for(int i=0;i<m;i++){
+            for(int j=0;j<n;j++){
+                board[i][j] = finalAns[i][j];
+            }
+        }
+    }
+
+
+    // DRIVER
+    public static void main(String args[]){
+
+        char board[][] = {
+            {'X','X','X','X'},
+            {'X','O','O','X'},
+            {'X','X','O','X'},
+            {'X','O','X','X'}
+        };
+
+        solve(board);
+
+        for(int i=0;i<board.length;i++){
+            for(int j=0;j<board[0].length;j++){
+                System.out.printf("%c ", board[i][j]);
+            }
+            System.out.printf("\n");
+        }
+    }
+}