class Solution {private:    int queen_num;    int total;public:    int totalNQueens(int n) {        queen_num = n;        total = 0;        vector<bool> h(n, false);        vector<bool> v(n, false);        vector<bool> lx(n * 2 + 1, false);        vector<bool> rx(n * 2 + 1, false);        dfs(0, h, v, lx, rx);        return total;    }    void dfs(int level, vector<bool> &h, vector<bool> &v, vector<bool> &lx, vector<bool> &rx) {        if (level == queen_num) {            total++;            return;        }        int row = level;        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;                dfs(level + 1, h, v, lx, rx);                h[row] = v[col] = lx[row + col] = rx[row + queen_num - col - 1] = false;            }            col++;        }    }};