public void solve(char[][] board) {

if (board == null || board.length == 0) return;

int m = board.length;

int n = board[0].length;

//寻找边界上的O，然后DFS深搜

for (int i = 0; i < m; i++) {

for (int j = 0; j < n; j++) {

boolean isEdge = i == 0 || j == 0 || i == m - 1 || j == n - 1;

if (isEdge && board[i][j] == 'O') {

dfs(board, i, j);

}

}

}

for (int i = 0; i < m; i++) {

for (int j = 0; j < n; j++) {

if (board[i][j] == 'O') {

board[i][j] = 'X';

}

if (board[i][j] == '#') {

board[i][j] = 'O';

}

}

}

}

public void dfs(char[][] board, int i, int j) {

if (i < 0 || j < 0 ||

i >= board.length || j >= board[0].length ||

board[i][j] == 'X' || board[i][j] == '#'

) {

// board[i][j] == '#' 说明已经搜索过了.

return;

}

board[i][j] = '#';

dfs(board, i - 1, j); // 上

dfs(board, i + 1, j); // 下

dfs(board, i, j - 1); // 左

dfs(board, i, j + 1); // 右

}