The n-queens puzzle is the problem of placing n queens on an n×n chessboard such that no two queens attack each other.

Given an integer n, return the number of distinct solutions to the n-queens puzzle.

Example:

Input: 4Output: 2Explanation: There are two distinct solutions to the 4-queens puzzle as shown below.[ [".Q..",  // Solution 1  "...Q",  "Q...",  "..Q."], ["..Q.",  // Solution 2  "Q...",  "...Q",  ".Q.."]]

 

AC code:

class Solution {public:    int totalNQueens(int n) {        int res = 0;        vector
     
      
       > v;        vector
       
         nqueens(n, string(n, '.'));        solve(v, nqueens, n, 0, res);        return res;    }         void solve(vector
        
         
          >& v, vector
          
           & nqueens, int n, int row, int& res) { if (row == n) { res++; return; } for (int col = 0; col < n; ++col) { if (judge(nqueens, n, row, col)) { nqueens[row][col] = 'Q'; solve(v, nqueens, n, row+1, res); nqueens[row][col] = '.'; } } } bool judge(vector
           
             nqueens, int n, int x, int y) { // up and down for (int i = 0; i < n && i != x; ++i) { if (nqueens[i][y] == 'Q') return false; } // right and left for (int i = 0; i < n && i != y; ++i) { if (nqueens[x][i] == 'Q') return false; } // left up for (int i = x-1, j = y-1; i >= 0 && j >= 0; --i, --j) { if (nqueens[i][j] == 'Q') return false; } // right up for (int i = x-1, j = y+1; i >= 0 && j < n; --i, ++j) { if (nqueens[i][j] == 'Q') return false; } return true; }};
           
          
         
        
       
      
     

Runtime: 16 ms, faster than 7.86% of C++ online submissions for N-Queens II.