剑指 Offer 59 – II. 队列的最大值

请定义一个队列并实现函数 max_value 得到队列里的最大值，要求函数max_value、push_back 和 pop_front 的均摊时间复杂度都是O(1)。

若队列为空，pop_front 和 max_value 需要返回 -1

示例 1：

输入:
["MaxQueue","push_back","push_back","max_value","pop_front","max_value"]
[[],[1],[2],[],[],[]]
输出: [null,null,null,2,1,2]

示例 2：

输入:
["MaxQueue","pop_front","max_value"]
[[],[],[]]
输出: [null,-1,-1]

限制：

• 1 <= push_back,pop_front,max_value的总操作数 <= 10000
• 1 <= value <= 10^5

Python 解答：

class MaxQueue:

    def __init__(self):
        self.alist = []
        self.blist = []

    def max_value(self) -> int:
        if not self.alist:
            return -1
        else:
            return self.blist[0]

    def push_back(self, value: int) -> None:
        self.alist.append(value)
        while self.blist and self.blist[-1] < value:
            self.blist.pop()
        self.blist.append(value)

    def pop_front(self) -> int:
        if not self.alist:
            return -1
        else:
            if self.alist[0] == self.blist[0]:
                self.blist.pop(0)
            return self.alist.pop(0)

# Your MaxQueue object will be instantiated and called as such:
# obj = MaxQueue()
# param_1 = obj.max_value()
# obj.push_back(value)
# param_3 = obj.pop_front()