Implement Queue using Stacks

Problem

Implement the following operations of a queue using stacks.

push(x) -- Push element x to the back of queue. pop() -- Removes the element from in front of queue. peek() -- Get the front element. empty() -- Return whether the queue is empty. 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, stack may not be supported natively. You may simulate a stack by using a list or deque (double-ended queue), as long as you use only standard operations of a stack. You may assume that all operations are valid (for example, no pop or peek operations will be called on an empty queue).

Solution

O(1) Solution

public class MyQueue {

    private Stack<Integer> s1;
    private Stack<Integer> s2;
    private int front; //Just for the peek()

    /** Initialize your data structure here. */
    public MyQueue() {
        this.s1 = new Stack<>();
        this.s2 = new Stack<>();
    }

    /** Push element x to the back of queue. */
    public void push(int x) {
        if (s1.isEmpty()) front = x;
        s1.push(x);
    }

    /** Removes the element from in front of queue and returns that element. */
    public int pop() {
        if (s2.isEmpty()) {
            while (!s1.isEmpty()) s2.push(s1.pop());
        }
        return s2.pop();
    }

    /** Get the front element. */
    public int peek() {
        if (!s2.isEmpty()) return s2.peek();
        return front;
    }

    /** Returns whether the queue is empty. */
    public boolean empty() {
        return s1.isEmpty() && s2.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();
 */

Analysis

The key idea is to use two stacks and a variable front to mark the beginning of queue
In the push() method, we set front if there s1.isEmpty()
In the pop() method, we move numbers from s1 to s2 to make it FIFO
In the peek() method, we first check s2, if it's empty we return the front
In the empty() method, we simply return the emptiness of s1 & s2