Implement Queue using Stacks

class MyQueue {    // Using two stacks.    Stack<Integer> s1 = new Stack<Integer>();// Pop out.    Stack<Integer> s2 = new Stack<Integer>();// Cache.        // Push element x to the back of queue.    public void push(int x) {        s2.push(x);    }    // Removes the element from in front of queue.    public void pop() {        if (empty()) {            return;        }        if (s1.isEmpty()) {            while (!s2.isEmpty()) {                s1.push(s2.peek());                s2.pop();            }        }        s1.pop();    }    // Get the front element.    public int peek() {        if (empty()) {            return -1;        }        if (s1.isEmpty()) {            while (!s2.isEmpty()) {                s1.push(s2.peek());                s2.pop();            }        }        return s1.peek();    }    // Return whether the queue is empty.    public boolean empty() {        return s1.isEmpty() && s2.isEmpty();    }}