N皇后问题

def solve(____):
    n = int(____)
    solutions = []
    def bt(____):
        if row == n:
            solutions.append(['____'.join(____) for r in board])
            return
        for col in range(____):
            if col in cols or (____) in diag1 or (____) in diag2:
                continue
            board[row][col] = '____'
            bt(____)
            board[row][col] = '____'
    board = [['____']*n for _ in range(____)]
    bt(____, set(), set(), set(), board)
    print(len(____))