用python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。

代码如下：

class Solution:
    def solveNQueens(self, n: int) -> List[List[str]]:
        def backtrack(row):
            if row == n:
                self.count += 1
                if self.count <= 3:
                    self.res.append(["".join(row) for row in board])
                if self.count == self.k:
                    self.result.append(["".join(row) for row in board])
                return
            for col in range(n):
                if not isValid(row, col):
                    continue
                board[row][col] = "Q"
                cols.add(col)
                diagonals.add(row - col)
                anti_diagonals.add(row + col)
                backtrack(row + 1)
                board[row][col] = "."
                cols.remove(col)
                diagonals.remove(row - col)
                anti_diagonals.remove(row + col)

        def isValid(row, col):
            if col in cols or row - col in diagonals or row + col in anti_diagonals:
                return False
            return True

        self.count = 0
        self.res = []
        self.result = []
        self.k = 3
        board = [["."] * n for _ in range(n)]
        cols = set()
        diagonals = set()
        anti_diagonals = set()
        backtrack(0)
        print("总共有{}个解:".format(self.count))
        print(self.res)
        print("前{}个解:".format(self.k))
        print(self.result)
        print("后{}个解:".format(self.k))
        print(self.result[-self.k:])
        return self.result

输出截图如下：


其中，第一行输出了总共的解数，第二行输出了前3个解，第三行输出了后3个解。