Leetcode: Implement Stack using Queues

Question

Implement the following operations of a stack using queues.

push(x) – Push element x onto stack.

pop() – Removes the element on top of the stack.

top() – Get the top element.

empty() – Return whether the stack is empty.

Notes:

You must use only standard operations of a queue – which means only push to back, peek/pop from front, size, and is empty operations are valid.

Depending on your language, queue may not be supported natively. You may simulate a queue by using a list or deque (double-ended queue), as long as you use only standard operations of a queue.

You may assume that all operations are valid (for example, no pop or top operations will be called on an empty stack).

Update (2015-06-11):

The class name of the Java function had been updated to MyStack instead of Stack.

Credits:

Special thanks to @jianchao.li.fighter for adding this problem and all test cases.

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Solution

Code

[code]class Stack(object):
    def __init__(self):
        """
        initialize your data structure here.
        """
        self.queue1 = []
        self.queue2 = []

    def push(self, x):
        """
        :type x: int
        :rtype: nothing
        """
        self.queue1.append(x)

    def pop(self):
        """
        :rtype: nothing
        """

        while len(self.queue1)>1:
            temp = self.queue1[0]
            self.queue2.append(temp)
            self.queue1 = self.queue1[1:] 

        self.queue1 = self.queue2[:]
        self.queue2 = []

    def top(self):
        """
        :rtype: int
        """

        res = None
        while len(self.queue1)>1:
            temp = self.queue1[0]
            self.queue2.append(temp)
            self.queue1 = self.queue1[1:]         

        if self.queue1!=[]:
            res = self.queue1[0]
            self.queue2.append(res)

        self.queue1 = self.queue2[:]
        self.queue2 = []   

        return res

    def empty(self):
        """
        :rtype: bool
        """

        return len(self.queue1)==0 and len(self.queue2)==0