232. Implement Queue using Stacks

Published on May 9, 2023

Question

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).

Algorithm

This question is an implementation problem. For a queue, you need to maintain the FIFO. So the oldest element will be the last out, but stack can only pop the newest element.

So we need an additional container to help. This is the implementation: You "pour" all the element to the other container and put the newest to the stack bottom, and "pour" back the other elements. So the elements in the stack will like a reversed queue and can be popped in right order;

Code

class MyQueue {
    Stack<Integer> stack1;
    Stack<Integer> stack2;

    public MyQueue() {
        stack1 = new Stack();
        stack2 = new Stack();
    }
    
    public void push(int x) {
        while (!stack1.isEmpty()) {
            stack2.push(stack1.pop());
        }
        stack2.push(x);
        while (!stack2.isEmpty()) {
            stack1.push(stack2.pop());
        }
    }
    
    public int pop() {
        return stack1.pop();
    }
    
    public int peek() {
        return stack1.peek();
    }
    
    public boolean empty() {
        return stack1.isEmpty();
    }
}

/**
 * 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();
 */