class CQueue:
    """
    用两个栈实现一个队列。队列的声明如下，请实现它的两个函数 appendTail 和 deleteHead ，
    分别完成在队列尾部插入整数和在队列头部删除整数的功能。(若队列中没有元素，deleteHead 操作返回 -1 )
    """

    def __init__(self):
        # A for pushing element, B for dequeuing
        self.A = []
        self.B = []


    def appendTail(self, value: int) -> None:
        self.A.append(value)


    def deleteHead(self) -> int:
        if self.B:
            return self.B.pop() # B is not empty, sequentially dequeue
        if not self.A:
            return -1 # A and B are all empty
        while self.A:
            self.B.append(self.A.pop()) # Transfer elements from A to B
        return self.B.pop() # B is not empty



# Your CQueue object will be instantiated and called as such:
# obj = CQueue()
# obj.appendTail(value)
# param_2 = obj.deleteHead()
# 输入：
# ["CQueue","deleteHead","appendTail","appendTail","deleteHead","deleteHead"]
# [[],[],[5],[2],[],[]]
# 输出：[null,-1,null,null,5,2]