LeetCode 51 n皇后

const int N = 20;

class Solution {
public:
    
    vector<vector<string>> res;
    bool col[N], dg[N], udg[N];

    void dfs(int u, int n, vector<string>& path) {
        if (u == n) {   
            res.push_back(path);
            return;
        }

        for (int i = 0; i < n; i ++) {
            if (!col[i] && !dg[u + i] && !udg[n - u + i]) {
                path[u][i] = 'Q';
                col[i] = dg[u + i] = udg[n - u + i] = true;
                dfs(u + 1, n, path);
                path[u][i] = '.';
                col[i] = dg[u + i] = udg[n - u + i] = false;
            }
        }
    }

    vector<vector<string>> solveNQueens(int n) {
        res.clear();
        vector<string> path(n, string(n, '.')); // 初始化**
        dfs(0, n, path);

        return res;
    }
};