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.


MyStack stack = new MyStack();

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


  • 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).


1. Queue

# Time: O(n)
# Space: O(n)
class MyStack:

    def __init__(self):
        Initialize your data structure here.
        self.queue = []

    def push(self, x: int) -> None:
        Push element x onto stack.
        self.queue.insert(0, x)
        n = len(self.queue)
        for _ in range(n-1):
            self.queue.insert(0, self.queue.pop())

    def pop(self) -> int:
        Removes the element on top of the stack and returns that element.
        return self.queue.pop()

    def top(self) -> int:
        Get the top element.
        return self.queue[-1]

    def empty(self) -> bool:
        Returns whether the stack is empty.
        return not any(self.queue)

# Your MyStack object will be instantiated and called as such:
# obj = MyStack()
# obj.push(x)
# param_2 = obj.pop()
# param_3 =
# param_4 = obj.empty()
# 16/16 cases passed (32 ms)
# Your runtime beats 51.75 % of python3 submissions
# Your memory usage beats 100 % of python3 submissions (12.7 MB)


  1. 225. Implement Stack using Queues