N queens ii

class Solution:
    def totalNQueens(self, n: int) -> int:
        res  = []
        grid = [['.'] * n for i in range(n)]

        def check(grid, row, col):
            for i in range(row - 1, -1, -1):
                if grid[i][col] == "Q":
                    return False

            count = 1
            for i in range(row - 1, -1, -1):
                leftcol = col - count
                rightcol = col + count
                if leftcol >= 0 and grid[i][leftcol] == "Q":
                    return False
                if rightcol < n and grid[i][rightcol] == "Q":
                    return False
                count += 1

            return True


        def backtrack(grid, row):
            # print(grid)
            if row == n:
                res.append(["".join(i) for i in grid])
                return

            for col in range(n):
                grid[row][col] = 'Q'
                if check(grid, row, col):
                    backtrack(grid, row + 1)
                grid[row][col] = '.'

        backtrack(grid, 0)

        return len(res)

N-Queens II

Difficulty: Hard

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