python - 不使用堆栈的反向队列

Question

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

    def __str__(self):
        return "Node({})".format(self.value)

    __repr__ = __str__

class Queue:
    def __init__(self):
        self.head=None
        self.tail=None

    def __str__(self):
        temp=self.head
        out=[]
        while temp:
            out.append(str(temp.value))
            temp=temp.next
        out=' '.join(out)
        return ('Head:{}\nTail:{}\nQueue:{}'.format(self.head,self.tail,out))

    __repr__=__str__

    def isEmpty(self):
        #write your code here
        return (self.head == None) and (self.tail == None)
    def __len__(self):
        #write your code here
        current = self.head
        len = 0
        while current:
            len += 1
            current = self.head.next
            if self.head.next == self.tail:
                break
        return len

    def enqueue(self, value):
        #write your code here
        node = Node(value)
        if self.isEmpty():
            self.head = node
            self.tail = node
        else:
            self.tail.next = node
            self.tail = node

    def dequeue(self):
        #write your code here
        if self.isEmpty():
            return 'Queue is empty'
        else:
            popped = self.head.value
            self.head = self.head.next
            return popped

    def front(self):
        if self.head != None:

            return self.head.value

q = Queue()
def reversequeue(q):
    if q.isEmpty() == False:        
        data = q.front()
        q.dequeue()
        reversequeue(q)
        q.enqueue(data)
def QueuePalindrome(word):
    q = Queue()
    for i in word:
        q.enqueue(i)

    p = reversequeue(q)
    return p

函数reversequeue返回“Nonetype”对象没有属性值。我不知道如何继续。

当我测试 q.front() 时。它返回正确的答案。我尝试通过指出如果 self.head == None 则跳过它来捕获错误。

遇到这种情况我该怎么办？

请帮忙。非常感谢!

Answer 1

您的函数reversequeue(q)没有return语句

def reversequeue(q):
    if q.isEmpty() == False:        
        data = q.front()
        q.dequeue()
        reversequeue(q)
        q.enqueue(data)

因此它默认返回None。只需为基本情况和递归情况添加返回，然后更新 q:

def reversequeue(q):
    if q.isEmpty() == False:        
        data = q.front()
        q.dequeue()
        q = reversequeue(q)
        q.enqueue(data)
        return q
    return Queue()