[LeetCode] Surrounded Regions

Thought: Any point that could not be reached if we started from four edges should be painted as 'X'.

Code:
public class Solution {
    public void solve(char[][] board) {
        int rows = board.length;
        if (rows == 0) return;
        int cols = board[0].length;
        for (int j = 0; j < cols; j++) markFrom(board, 0, j);
        for (int i = 0; i < rows; i++) markFrom(board, i, 0);
        for (int j = 0; j < cols; j++) markFrom(board, rows - 1, j);
        for (int i = 0; i < rows; i++) markFrom(board, i, cols - 1);
        for (int i = 0; i < rows; i++) {
            for (int j = 0; j < cols; j++) {
                if (board[i][j] == 'M') board[i][j] = 'O';
                else board[i][j] = 'X';
            }
        }    
    }
    public void markFrom(char[][] board, int x, int y) {
        int rows = board.length;
        int cols = board[0].length;
        if (x < 0 || x >= rows) return;
        if (y < 0 || y >= cols) return;
        if (board[x][y] == 'X' || board[x][y] == 'M') return;
        board[x][y] = 'M';
        markFrom(board, x - 1, y);
        markFrom(board, x, y - 1);
        markFrom(board, x + 1, y );
        markFrom(board, x, y + 1);
    }
}