面试题09. 用两个栈实现队列

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 )

示例 1：

输入：
["CQueue","appendTail","deleteHead","deleteHead"]
[[],[3],[],[]]
输出：[null,null,3,-1]
示例 2：

输入：
["CQueue","deleteHead","appendTail","appendTail","deleteHead","deleteHead"]
[[],[],[5],[2],[],[]]
输出：[null,-1,null,null,5,2]

class CQueue:

    def __init__(self):
        self.s1 = []
        self.s2 = []
        self.size = 0

    def appendTail(self, value: int) -> None:
        self.s1.append(value)
        self.size += 1

    def deleteHead(self) -> int:
        if(0 == self.size):
            return -1
        
        if self.s2 == []:
            while self.s1 != []:
                self.s2.append(self.s1.pop())
        
        self.size -= 1
        return self.s2.pop()


# Your CQueue object will be instantiated and called as such:
# obj = CQueue()
# obj.appendTail(value)
# param_2 = obj.deleteHead()