回溯：N皇后问题

def solve():
    def is_valid(____):
        for i in range(____):
            if board[i] == col:
                return False
            if abs(____) == abs(____):
                return False
        return True
    def backtrack(____):
        if row == ____:
            result.append(____)
            return
        for col in range(____):
            if is_valid(____):
                board.append(____)
                backtrack(____)
                board.pop()
    result = []
    backtrack(____, [])
    print(len(____))