N-Queens II

 1 class Solution {
 2 public:
 3     int totalNQueens( int n ) {
 4         solutions = 0;
 5         vector<string> solution( n, string( n, ‘.‘ ) );
 6         SolveSub( solution, 0, n );
 7         return solutions;
 8     }
 9 private:
10     void SolveSub( vector<string>& solution, int step, const int &n ) {
11         if( step == n ) { ++solutions; return; }
12         for( int i = 0; i < n; ++i ) {
13             bool used = false;
14             for( int k = step-1; k >= 0; --k ) {
15                 if( solution[k][i] == ‘Q‘ ) { used = true; break; }
16             }
17             if( used ) { continue; }
18             for( int k = 1; k <= min( i, step ); ++k ) {
19                 if( solution[step-k][i-k] == ‘Q‘ ) { used = true; break; }
20             }
21             if( used ) { continue; }
22             for( int k = 1; k <= min( n-i, step ); ++k ) {
23                 if( solution[step-k][i+k] == ‘Q‘ ) { used = true; break; }
24             }
25             if( used ) { continue; }
26             solution[step][i] = ‘Q‘;
27             SolveSub( solution, step+1, n );
28             solution[step][i] = ‘.‘;
29         }
30         return;
31     }
32     int solutions;
33 };