1 class Solution { 2 public: 3     vector<vector<string> > solveNQueens(int n) { 4      vector<vector<string>> res; 5      vector<vector<string>> pre_res; 6      for(int i = 0; i < n; i++){ 7          res.clear(); 8          for(int j = 0; j < n; j++){ 9              if(i == 0){10                  vector<string> tmp;11                  string s;12                  s.assign(n,‘.‘);13                  s[j] = ‘Q‘;14                  tmp.push_back(s);15                  res.push_back(tmp);16              }else{17                  for(int k = 0; k < pre_res.size(); k++){18                      if(not_attack(pre_res[k],i,j,n)){19                          string s;20                          s.assign(n,‘.‘);21                          s[j] = ‘Q‘;22                          vector<string> tmp;23                          tmp = pre_res[k];24                          tmp.push_back(s);25                          res.push_back(tmp);26                      }27                  }28                  29              }30          }31          pre_res = res;32      }33      return pre_res;   34     }35     bool not_attack(vector<string> cb, int row,int col, int n){36         int i = 0;37         while(i < row){38             if(cb[i][col] == ‘Q‘) return false;39             i++;40         }41         i = row - 1;42         int j = col - 1;43         while(i >= 0 && j >= 0){44             if(cb[i][j] == ‘Q‘) return false;45             i--;46             j--;47         }48         i = row - 1;49         j = col + 1;50         while(i >= 0 && j < n){51             if(cb[i][j] == ‘Q‘) return false;52             i--;53             j++;54         }55         return true;56     }57 };