52. N-Queens II刷题笔记

回溯算法，参考该题解

class Solution:
    def totalNQueens(self, n: int) -> int:
        diag1 = set()
        diag2 = set()
        usedCols = set()
        
        return self.helper(n, diag1, diag2, usedCols, 0)

    def helper(self, n, diag1, diag2, usedCols, row):
        if row == n:
            return 1
        
        solutions = 0
        
        for col in range(n):
            if row + col in diag1 or row - col in diag2 or col in usedCols:
                continue
                
            diag1.add(row + col)
            diag2.add(row - col)
            usedCols.add(col)
            
            solutions += self.helper(n, diag1, diag2, usedCols, row + 1)
        
            diag1.remove(row + col)
            diag2.remove(row - col)
            usedCols.remove(col)
        
        return solutions