用回溯法实现8皇后问题，输出所有可能的解的总数，并展示前3个与后3个解，将代码及输出截图提交。

以下是Python实现的代码：

class Solution:
    def __init__(self):
        self.res = []
    
    def solveNQueens(self, n: int) -> List[List[str]]:
        board = [['.' for _ in range(n)] for _ in range(n)]
        self.backtrack(board, 0)
        return self.res
    
    def backtrack(self, board, row):
        if row == len(board):
            self.res.append([''.join(row) for row in board])
            return
        n = len(board[row])
        for col in range(n):
            if not self.isValid(board, row, col):
                continue
            board[row][col] = 'Q'
            self.backtrack(board, row+1)
            board[row][col] = '.'
    
    def isValid(self, board, row, col):
        for i in range(row):
            if board[i][col] == 'Q':
                return False
        for i, j in zip(range(row-1, -1, -1), range(col-1, -1, -1)):
            if board[i][j] == 'Q':
                return False
        for i, j in zip(range(row-1, -1, -1), range(col+1, len(board))):
            if board[i][j] == 'Q':
                return False
        return True

输出所有可能的解的总数：


展示前3个与后3个解：