class Solution {

 public:

     /**

      * Get all distinct N-Queen solutions

      * @param n: The number of queens

      * @return: All distinct solutions

      * For example, A string '...Q' shows a queen on forth position

      */

     bool isOK(vector<string> &v, int n, int x, int y) {

         for (int i = ; i < x; ++i)

             if (v[i][y] == 'Q') return false;

         for (int i = ; x - i >=  && y - i >= ; ++i)

             if (v[x-i][y-i] == 'Q') return false;

         for (int i = ; x - i >=  && y + i < n; ++i)

             if (v[x-i][y+i] == 'Q') return false;

         return true;

     }

     void dfs(vector<vector<string>> &res, vector<string> &v, int n, int idx) {

         if (idx == n) {

             res.push_back(v);

             return;

         }

         for (int i = ; i < n; ++i) {

             v[idx][i] = 'Q';

             if (isOK(v, n, idx, i)) dfs(res, v, n, idx + );

             v[idx][i] = '.';

         }

     }

     vector<vector<string> > solveNQueens(int n) {

         // write your code here

         vector<vector<string>> res;

         string s;

         for (int i = ; i < n; ++i) s.push_back('.');

         vector<string> v(n, s);

         dfs(res, v, n, );

         return res;

     }

 };