LeetCode52 N-Queens II

 1 class Solution {
 2 private:
 3     int result = 0;
 4     void helper(vector<string>& v, int row, int n) {
 5         for (int i = 0; i < n; ++i) {
 6             v[row][i] = ‘Q‘;
 7             if (isValid(v, row, i, n)) {
 8                 if (row == n - 1) {
 9                     result++;
10                     v[row][i] = ‘.‘;
11                     return;
12                 }
13                 helper(v, row + 1, n);
14             }
15             v[row][i] = ‘.‘;
16         }
17     }
18     bool isValid (const vector<string>& v, int x, int y, int n) {
19         for (int i = 0; i < x; ++i) {
20             if (v[i][y] == ‘Q‘) {
21                 return false;
22             }
23         }
24         for(int i = x - 1, j = y - 1; i >= 0 && j >= 0; i--,j--) {
25             if(v[i][j] == ‘Q‘) {
26                 return false;
27             }
28         }
29         for(int i = x - 1, j = y + 1; i >= 0 && j < n; i--,j++){
30             if(v[i][j] == ‘Q‘) {
31                 return false;
32             }
33         }
34         return true;
35     }
36 public:
37     int totalNQueens(int n) {
38         vector<string> v(n, string(n, ‘.‘));
39         helper(v,0,n);
40         return result;
41     }
42 };