class Solution {private:    int queen_num;    vector<vector<string> > res;public:    vector<vector<string> > solveNQueens(int n) {        res.clear();        queen_num = n;                vector<bool> h(n, false);        vector<bool> v(n, false);        vector<bool> lx(n * 2 + 1, false);        vector<bool> rx(n * 2 + 1, false);        vector<int> solution;                dfs(solution, h, v, lx, rx);                return res;    }    void dfs(vector<int> &mem, vector<bool> &h, vector<bool> &v, vector<bool> &lx, vector<bool> &rx) {        if (mem.size() == queen_num) {            add_solution(mem);            return;        }        int row = mem.size();        int col = 0;        while (col < queen_num) {            if (!h[row] && !v[col] && !lx[row + col] && !rx[row + queen_num - col - 1]) {                h[row] = v[col] = lx[row + col] = rx[row + queen_num - col - 1] = true;                mem.push_back(col);                dfs(mem, h, v, lx, rx);                mem.pop_back();                h[row] = v[col] = lx[row + col] = rx[row + queen_num - col - 1] = false;            }            col++;        }    }    void add_solution(vector<int> &solution) {        res.push_back(vector<string>());        string line(queen_num, ‘.‘);        for (int i=0; i<queen_num; i++) {            line[solution[i]] = ‘Q‘;            res.back().push_back(line);            line[solution[i]] = ‘.‘;        }    }};