题目：

题解：

class Solution {
public:const int dx[4] = {1, -1, 0, 0};const int dy[4] = {0, 0, 1, -1};void solve(vector<vector<char>>& board) {int n = board.size();if (n == 0) {return;}int m = board[0].size();queue<pair<int, int>> que;for (int i = 0; i < n; i++) {if (board[i][0] == 'O') {que.emplace(i, 0);board[i][0] = 'A';}if (board[i][m - 1] == 'O') {que.emplace(i, m - 1);board[i][m - 1] = 'A';}}for (int i = 1; i < m - 1; i++) {if (board[0][i] == 'O') {que.emplace(0, i);board[0][i] = 'A';}if (board[n - 1][i] == 'O') {que.emplace(n - 1, i);board[n - 1][i] = 'A';}}while (!que.empty()) {int x = que.front().first, y = que.front().second;que.pop();for (int i = 0; i < 4; i++) {int mx = x + dx[i], my = y + dy[i];if (mx < 0 || my < 0 || mx >= n || my >= m || board[mx][my] != 'O') {continue;}que.emplace(mx, my);board[mx][my] = 'A';}}for (int i = 0; i < n; i++) {for (int j = 0; j < m; j++) {if (board[i][j] == 'A') {board[i][j] = 'O';} else if (board[i][j] == 'O') {board[i][j] = 'X';}}}}
};