232. Implement Queue using Stacks

Implement the following operations of a queue using stacks.

• push(x) -- Push element x to the back of queue.
• pop() -- Removes the element from in front of queue.
• peek() -- Get the front element.
• empty() -- Return whether the queue is empty.

Notes:

• You must use only standard operations of a stack -- which means only push to top, peek/pop from top, size, and is empty operations are valid.
• Depending on your language, stack may not be supported natively. You may simulate a stack by using a list or deque (double-ended queue), as long as you use only standard operations of a stack.
• You may assume that all operations are valid (for example, no pop or peek operations will be called on an empty queue).

Solution: Stack

class MyQueue(object):
    def __init__(self):
        """
        Initialize your data structure here.
        """
        self.stack = []
        self.head = None

    def push(self, x):
        """
        Push element x to the back of queue.
        :type x: int
        :rtype: void
        """
        if not self.stack:
            self.head = x
        self.stack.append(x)

    def pop(self):
        """
        Removes the element from in front of queue and returns that element.
        :rtype: int
        """
        temp = []
        while self.stack:
            temp.append(self.stack.pop())
        head = temp.pop()
        self.head = temp[-1] if temp else None
        while temp:
            self.stack.append(temp.pop())
        return head

    def peek(self):
        """
        Get the front element.
        :rtype: int
        """
        return self.head

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