一、题目

The n-queens puzzle is the problem of placing n queens on an n x 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 1:

Input: n = 4
Output: 2
Explanation: There are two distinct solutions to the 4-queens puzzle as shown.
Example 2:

Input: n = 1
Output: 1

Constraints:

1 <= n <= 9

二、题解

class Solution {
public:int res;bool isValid(vector<string>& chessboard,int row,int col,int n){//检查列for(int i = 0;i < row;i++){if(chessboard[i][col] == 'Q') return false;}//检查45度角for(int i = row - 1,j = col - 1;i >= 0 && j >= 0;i--,j--){if(chessboard[i][j] == 'Q') return false;}//检查135度角for(int i = row - 1,j = col + 1;i >= 0 && j < n;i--,j++){if(chessboard[i][j] == 'Q') return false;}return true;}void backtracing(vector<string>& chessboard,int row,int n){if(row == n){res++;return;}for(int i = 0;i < n;i++){if(isValid(chessboard,row,i,n)){chessboard[row][i] = 'Q';backtracing(chessboard,row+1,n);chessboard[row][i] = '.';}}}int totalNQueens(int n) {vector<string> chessboard(n,string(n,'.'));backtracing(chessboard,0,n);return res;}
};