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.

Example:

MyStack stack = new MyStack();

stack.push(1);
stack.push(2);  
stack.top();   // returns 2
stack.pop();   // returns 2
stack.empty(); // returns false

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).

Solution

Use the two queues to find the last item.

from collections import deque

class MyStack(object):

    def __init__(self):
        """
        Initialize your data structure here.
        """
        self.inque = deque()
        self.outque = deque()


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


    def pop(self):
        """
        Removes the element on top of the stack and returns that element.
        :rtype: int
        """
        while len(self.inque) > 1:
            n = self.inque.popleft()
            self.outque.append(n)

        out = self.inque.popleft()
        self.inque, self.outque = self.outque, self.inque
        return out


    def top(self):
        """
        Get the top element.
        :rtype: int
        """
        if len(self.inque) == 0:
            raise IndexError
        else:
            return self.inque[-1]


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