Implement Queue using Stacks LeetCode Solution – Queslers

Problem – 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.

Follow-up: Can you implement the queue such that each operation is amortizedO(1) time complexity? In other words, performing n operations will take overall O(n) time even if one of those operations may take longer.

Implement Queue using Stacks LeetCode Solution in Java

class MyQueue {

    Stack<Integer> input = new Stack();
    Stack<Integer> output = new Stack();
    public void push(int x) {

    public void pop() {

    public int peek() {
        if (output.empty())
            while (!input.empty())
        return output.peek();

    public boolean empty() {
        return input.empty() && output.empty();

Implement Queue using Stacks LeetCode Solution in C++

class Queue {
    stack<int> input, output;

    void push(int x) {

    void pop(void) {

    int peek(void) {
        if (output.empty())
            while (input.size())
                output.push(, input.pop();

    bool empty(void) {
        return input.empty() && output.empty();

Implement Queue using Stacks LeetCode Solution in Python

class Queue(object):
    def __init__(self):
        initialize your data structure here.
        self.inStack, self.outStack = [], []

    def push(self, x):
        :type x: int
        :rtype: nothing

    def pop(self):
        :rtype: nothing

    def peek(self):
        :rtype: int
        return self.outStack[-1]

    def empty(self):
        :rtype: bool
        return (not self.inStack) and (not self.outStack) 
    def move(self):
        :rtype nothing
        if not self.outStack:
            while self.inStack:
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