两个栈实现队列

核心是adjust方法：
每次pop或top就是更新时间，更新前提是辅助栈为空

 def __init__(self):
        # do intialization if necessary
        self.stack_main = []
        self.stack_help = []

    """
    @param: element: An integer
    @return: nothing
    """
    def push(self, element):
        # write your code here
        self.stack_main.append(element)

    """
    @return: An integer
    """
    def adjust(self):
        if len(self.stack_help) == 0:
            while len(self.stack_main) != 0:
                self.stack_help.append(self.stack_main.pop(-1))
    def pop(self):
        # write your code here
        self.adjust()
        n = len(self.stack_help)
        if n == 0:
            return None
        
        return self.stack_help.pop(-1)
       

    """
    @return: An integer
    """
    def top(self):
        # write your code here
        self.adjust()
        n = len(self.stack_help)
        if n == 0:
            return None
        return self.stack_help[-1]