232. Implement Queue using Stacks

mplement 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.

Example:

MyQueue queue = new MyQueue();

queue.push(1);
queue.push(2);  
queue.peek();  // returns 1
queue.pop();   // returns 1
queue.empty(); // returns false

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).

class MyQueue {
    Stack<Integer> s1;
    Stack<Integer> s2;
    
    /** Initialize your data structure here. */
    public MyQueue() {
        s1 = new Stack<Integer>();
        s2 = new Stack<Integer>();
    }
    
    /** Push element x to the back of queue. */
    public void push(int x) { //Time: O(n)
        while (!s1.isEmpty()) {
            s2.push(s1.pop());
        }
        
        s1.push(x);
        
        while (!s2.isEmpty()) {
            s1.push(s2.pop());
        }
    }
    
    /** Removes the element from in front of queue and returns that element. */
    public int pop() {//Time: O(1)
        return s1.pop();
    }
    
    /** Get the front element. */
    public int peek() {//Time: O(1)
        return s1.peek();
    }
    
    /** Returns whether the queue is empty. */
    public boolean empty() {//Time: O(1)
        return s1.isEmpty();
    }
}