队列

class Node:
    def __init__(self, data):
        self.data = data
        self.next = None

class Queue:
    def __init__(self):
        self.__head = None
        self.__tail = None

    def push(self, data):
        node = Node(data)
        if not self.__head:
            self.__head = node
        if self.__tail:
            self.__tail.next = node
        self.__tail = node

    def pop(self):
        if not self.__head:
            raise Exception("队列是空的！")
        returnedValue = self.__head.data
        self.__head = self.__head.next
        if not self.__head:
            self.__tail = None
        return returnedValue

    def empty(self):
        return not bool(self.__head)

    def __len__(self):
        count = 0
        temp = self.__head
        while temp:
            count += 1
            temp = temp.next
        return count

    def front(self):
        if not self.__head:
            raise Exception("队列是空的！")
        return self.__head.data

    def back(self):
        if not self.__tail:
            raise Exception("队列是空的！")
        return self.__tail.data

queue = Queue()
queue.push(1)
queue.push(4)
queue.push(5)
queue.push(7)
queue.push(9)
print(len(queue))
print(queue.front())
print(queue.back())
print(queue.pop())
print(queue.pop())
print(queue.empty())
print(queue.pop())
print(queue.pop())
print(queue.pop())
print(queue.pop())