225. Implement Stack using Queues

Implement a last in first out (LIFO) stack using only two queues. The implemented stack should support all the functions of a normal queue (push, top, pop, and empty).

Implement the MyStack class:

  • void push(int x) Pushes element x to the top of the stack.
  • int pop() Removes the element on the top of the stack and returns it.
  • int top() Returns the element on the top of the stack.
  • boolean empty() Returns true if the stack is empty, false otherwise.


  • 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, the queue may not be supported natively. You may simulate a queue using a list or deque (double-ended queue), as long as you use only a queue's standard operations.


Example 1:

["MyStack", "push", "push", "top", "pop", "empty"]
[[], [1], [2], [], [], []]
[null, null, null, 2, 2, false]

MyStack myStack = new MyStack();
myStack.top(); // return 2
myStack.pop(); // return 2
myStack.empty(); // return False



  • 1 <= x <= 9
  • At most 100 calls will be made to push, pop, top, and empty.
  • All the calls to pop and top are valid.


Follow-up: Can you implement the stack such that each operation is amortized O(1) time complexity? In other words, performing n operations will take overall O(n) time even if one of those operations may take longer. You can use more than two queues.

225. Implement Stack using Queues
use std::collections::VecDeque;

struct MyStack {
    queue: VecDeque<i32>,

impl MyStack {
    fn new() -> Self {
        MyStack {
            queue: VecDeque::new(),

    fn push(&mut self, x: i32) {
        let mut n = self.queue.len();
        while n > 0 {
            let front = self.queue.pop_front().unwrap();
            n -= 1;

    fn pop(&mut self) -> i32 {

    fn top(&self) -> i32 {

    fn empty(&self) -> bool {

fn test() {
    let mut stack = MyStack::new();
    assert_eq!(stack.top(), 2);
    assert_eq!(stack.pop(), 2);
    assert_eq!(stack.empty(), false);