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).

Tags

• Stack

Thought

It is similar to the previous problem, but the order in the queue is hard to reverse by transfering between the queues. However, we can still use this idea to implement the pop function. For the other parts, we can simply use a specific variable to represent the top and update it during pop and push.

Code

class MyStack(object):

    def __init__(self):
        """
        Initialize your data structure here.
        """
        self.queue = []
        self.topItem = None


    def push(self, x):
        """
        Push element x onto stack.
        :type x: int
        :rtype: void
        """
        self.queue.append(x)
        self.topItem = x


    def pop(self):
        """
        Removes the element on top of the stack and returns that element.
        :rtype: int
        """
        if self.queue == []:
            return None
        tmpQueue = []
        self.topItem = None
        while len(self.queue) > 1:
            self.topItem = self.queue.pop(0)
            tmpQueue.append(self.topItem)
        result = self.queue[0]
        self.queue = tmpQueue
        return result



    def top(self):
        """
        Get the top element.
        :rtype: int
        """
        return self.topItem



    def empty(self):
        """
        Returns whether the stack is empty.
        :rtype: bool
        """
        return len(self.queue) == 0



# Your MyStack object will be instantiated and called as such:
# obj = MyStack()
# obj.push(x)
# param_2 = obj.pop()
# param_3 = obj.top()
# param_4 = obj.empty()