130-SurroundedRegions.java

// 130. Surrounded Regions
class Solution {
    int rows;
    int cols;
    
    public void solve(char[][] board) {
        
        if (board == null || board.length == 0)
            return;
        
        rows = board.length;
        cols = board[0].length;
        
        for(int i = 0; i < rows; i++) {
            if(board[i][0] == 'O')
                 dfs(board, i, 0);
            if(board[i][cols-1] == 'O')
                 dfs(board, i, cols-1);                        
        }
        
        for(int j = 0; j < cols; j++) {
            if(board[0][j] == 'O')
                dfs(board, 0, j);
            if(board[rows-1][j] == 'O')
                dfs(board, rows-1, j);
        }    
        
        for(int i = 0; i < rows; i++){
            for(int j = 0; j < cols; j++){
                if(board[i][j] == 'O')
                    board[i][j] = 'X';
                if(board[i][j] == 'E') 
                    board[i][j] = 'O';
            }
        }
    }
    
    private void dfs(char[][] board, int i, int j) {
        if(i < 0 || i >= rows || j < 0 || j >= cols || board[i][j] != 'O')
            return;
        board[i][j] = 'E';
        dfs(board, i-1, j);
        dfs(board, i+1, j);
        dfs(board, i, j-1);
        dfs(board, i, j+1);
        
    }
}