Merge pull request #218 from asong91/surrounded

Create 130-Surrounded-Regions.java

Author: neetcode-gh

Diff for: 130-Surrounded-Regions.java

@@ -0,0 +1,37 @@
+class Solution {
+    public void solve(char[][] board) {
+        int nRows = board.length;
+        int nCols = board[0].length;
+        
+        // 1a) Capture unsurrounded regions - top and bottom row (O -> T)
+        for (int i = 0; i < nCols; i++) {
+            if (board[0][i] == 'O') dfs(board, 0, i);
+            if (board[nRows-1][i] == 'O') dfs(board, nRows-1, i);
+        }
+        
+        // 1b) Capture unsurrounded regions - Left and right columns (O -> T)
+        for (int i = 0; i < nRows; i++) {
+            if (board[i][0] == 'O') dfs(board, i, 0);
+            if (board[i][nCols-1] == 'O') dfs(board, i, nCols-1);
+        }
+        
+        for (int r = 0; r < nRows; r++) {
+            for (int c = 0; c < nCols; c++) {
+                if (board[r][c] == 'O') board[r][c] = 'X'; // 2) Capture surrounded regions (O -> X)
+                if (board[r][c] == 'T') board[r][c] = 'O'; // 3) Uncapture unsurrounded regions (T- O)
+            }
+        }
+    }
+    
+    private void dfs(char[][] board, int r, int c) {
+        int nRows = board.length;
+        int nCols = board[0].length;
+        if (r < 0 || c < 0 || r >= nRows || c >= nCols || board[r][c] != 'O') return;
+        
+        board[r][c] = 'T';
+        dfs(board, r+1, c);
+        dfs(board, r-1, c);
+        dfs(board, r, c+1);
+        dfs(board, r, c-1);
+    }  
+}