[Leetcode]N-Queens II

Follow up for N-Queens problem.

Now, instead outputting board configurations, return the total number of distinct solutions.

class Solution {public:    /*algorithm: DFS    */    vector<int>getNextCandiates(vector<int>&solution,int n)      {          vector<int>result;          for(int i = 0; i < n;i++){             bool selected = true;             for(int k = 0;k < solution.size();k++){                 if(i == solution[k] ||  //same column or same k=1/-1 (k=(y2-y1)/(x2-x1))                    abs(i-solution[k])==abs((int)solution.size()-k)){                         selected = false;                        break;                    }             }             if(selected)result.push_back(i);          }          return result;      }           void dfs(int n,int start,int &sum,vector<int>&solution){          if(start == n){ //find one solution,reach last line              sum++;             return;          }          vector<int>candiates = getNextCandiates(solution,n);          for(int k = 0;k < candiates.size();k++){              solution.push_back(candiates[k]);              dfs(n,start+1,sum,solution);              solution.pop_back();          }      }      int totalNQueens(int n) {        vector<int>solution;          int sum = 0;          dfs(n,0,sum,solution);          return sum;            }};