52. N-Queens II(数个数)

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: 4
Output: 2
Explanation: 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.."]
]

 class Solution {
     private int cnt = 0;
     public int totalNQueens(int n) {
         char[][] board = new char[n][n];
         for(int i = 0; i < n; i++)
             for(int j = 0; j < n; j++)
                 board[i][j] = '.';
         dfs(board, 0);
         return cnt;

     }
     private void dfs(char[][] board, int col) {
         if(col==board.length){
             cnt++;
             return;
         }
         for(int i = 0;i<board.length;i++){
             if(validate(board,i,col)){
             board[i][col] = 'Q';
             dfs(board,col+1);
             board[i][col] = '.';
             }
         }
     }

     private boolean validate(char[][] board, int x, int y) {
        for(int i = 0;i<board.length;i++)
            for(int j = 0;j<y;j++)
                if(board[i][j]=='Q'&&(x==i||x+j==y+i||x+y==i+j))
                    return false;
         return true;
     }

 }