题目

法1：DFS

典型题目，牢记方法！

class Solution {
    public void solve(char[][] board) {
        int m = board.length, n = board[0].length;
        if (m == 0 || n == 0) {
            return;
        }
        boolean[][] visitedO = new boolean[m][n]; // 没有被围绕的'O'位置记录
        for (int i = 0; i < n; ++i) { // 首行
            dfs(board, visitedO, 0, i, m, n);
        }
        for (int i = 0; i < n; ++i) { // 末行
            dfs(board, visitedO, m - 1, i, m, n);
        }
        for (int i = 0; i < m; ++i) { // 首列
            dfs(board, visitedO, i, 0, m, n);
        }
        for (int i = 0; i < m; ++i) { // 末列
            dfs(board, visitedO, i, n - 1, m, n);
        }

        for (int i = 0; i < m; ++i) {
            for (int j = 0; j < n; ++j) {
                if (board[i][j] == 'O' && !visitedO[i][j]) {
                    board[i][j] = 'X';
                }
            }
        }
    }

    public void dfs(char[][] board, boolean[][] visitedO, int i, int j, int m, int n) {
        if (i >= 0 && i < m 
                && j >= 0 && j < n 
                && board[i][j] == 'O' 
                && !visitedO[i][j]) {
            visitedO[i][j] = true;
            dfs(board, visitedO, i - 1, j, m, n);
            dfs(board, visitedO, i + 1, j, m, n);
            dfs(board, visitedO, i, j - 1, m, n);
            dfs(board, visitedO, i, j + 1, m, n);
        }
    }
}