Day 39 of 100 Days LeetCode Challenge

LeetCode Challenge #232. Implement Queue using Stacks

Implement a first in first out (FIFO) queue using only two stacks. The implemented queue should support all the functions of a normal queue (pushpeekpop, and empty).

Implement the MyQueue class:

  • void push(int x) Pushes element x to the back of the queue.
  • int pop() Removes the element from the front of the queue and returns it.
  • int peek() Returns the element at the front of the queue.
  • boolean empty() Returns true if the queue is empty, false otherwise.


  • You must use only standard operations of a stack, which means only push to toppeek/pop from topsize, and is empty operations are valid.
  • Depending on your language, the stack may not be supported natively. You may simulate a stack using a list or deque (double-ended queue) as long as you use only a stack’s standard operations.


Example 1:

["MyQueue", "push", "push", "peek", "pop", "empty"]
[[], [1], [2], [], [], []]
[null, null, null, 1, 1, false]

MyQueue myQueue = new MyQueue();
myQueue.push(1); // queue is: [1]
myQueue.push(2); // queue is: [1, 2] (leftmost is front of the queue)
myQueue.peek(); // return 1
myQueue.pop(); // return 1, queue is [2]
myQueue.empty(); // return false



  • 1 <= x <= 9
  • At most 100 calls will be made to pushpoppeek, and empty.
  • All the calls to pop and peek are valid.
Video Solution
Java Solution
					class MyQueue {
     private Stack<Integer> main;
     private Stack<Integer> helper;

    public MyQueue() {
        main = new Stack<>();
        helper = new Stack<>();
    public void push(int x) {
        // add all elements of main to helper -- main --> helper 

        // add x to main 


        // add all elemets from helper to main -- helper ---> main 

    public int pop() {
        return main.pop();
    public int peek() {
        return main.peek();
    public boolean empty() {
        return main.size()==0;

 * Your MyQueue object will be instantiated and called as such:
 * MyQueue obj = new MyQueue();
 * obj.push(x);
 * int param_2 = obj.pop();
 * int param_3 = obj.peek();
 * boolean param_4 = obj.empty();

Happy Coding with edSlash