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:    def totalNQueens(self, n):        """        :type n: int        :rtype: List[List[str]]        """        board = [-1] *n        count = 0        def safe(a,b):            for i in range(a):                if board[i]==b or abs(board[i]-b)==abs(a-i):                    return False            return True        def dfs(line):            nonlocal count            if line == n:                count += 1                return            else:                for i in range(n):                    if safe(line,i):                        board[line] = i                        dfs(line + 1)        dfs(0)        return count

关键子nonlocal的作用与关键字global类似，使用nonlocal关键字可以在一个嵌套的函数中修改嵌套作用域中的变量