N-皇后 N-Queens

​​https://leetcode.com/problems/n-queens/​​

class Solution {
public:
    vector<vector<string>> solveNQueens(int n) {
        if (n <= 0){
            return res;
        }
        vector<int> sol(n);
        helper(n, sol, 0);
        return res;
    }

    void helper(int n,vector<int>& sol, int cur){
        if (cur == n){
            vector<string> vec;
            for (int i = 0; i < n; i++){
                string str = "";
                for (int j = 0; j < n; j++){
                    if (j != sol[i]){
                        str.append(".");
                    }
                    else{
                        str.append("Q");
                    }
                }
                vec.push_back(str);
            }
            res.push_back(vec);
            return;
        }

        for (int col = 0; col < n; col++){
            bool flag = true;
            for (int i = 0; i < cur; i++){
                if (sol[i] == col || sol[i] - i == col - cur || sol[i] + i == col + cur){
                    flag = false;
                    break;
                }
            }
            if (flag){
                sol[cur] = col;
                helper(n, sol, cur + 1);
            }
        }

    }

private:
    vector<vector<string> >res;
};