232. Implement Queue using Stacks

At a Glance

Topic: Stack
Pattern: Two Stacks
Difficulty: Easy
LeetCode: 232

Problem Statement

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

Notes:

You must use only standard operations of a stack, which means only push to top, peek/pop from top, size, 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&#39;s standard operations.

Example 1:

Input ["MyQueue", "push", "push", "peek", "pop", "empty"] [[], [1], [2], [], [], []] Output [null, null, nu...

Approach & Solution Steps

Use two stacks, one for enqueuing and one for dequeuing. When dequeuing, if the dequeue stack is empty, transfer all elements from the enqueue stack to reverse their order.

Optimal JS Solution

class MyQueue {
  constructor() {
    this.in = [];
    this.out = [];
  }
  push(x) {
    this.in.push(x);
  }
  pop() {
    this.peek();
    return this.out.pop();
  }
  peek() {
    if (this.out.length === 0) {
      while (this.in.length > 0) this.out.push(this.in.pop());
    }
    return this.out[this.out.length - 1];
  }
  empty() {
    return this.in.length === 0 && this.out.length === 0;
  }
}

Edge Cases & Pitfalls

Always consider empty or null inputs.
Watch out for off-by-one index errors.