Implement Queue using Stacks

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:
1. class MyQueue {
2. Stack s1 = new Stack<>();
3. Stack s2 = new Stack<>();
4. 5. // Push element x to the back of queue.
6. public void push(int x) {
7. s1.push(x);
8. }
9. 10. // Removes the element from in front of queue.
11. public void pop() {
12. peek();
13. s2.pop();
14. }
15. 16. // Get the front element.
17. public int peek() {
18. if (s2.isEmpty()) {
19. while (!s1.isEmpty()) {
20. s2.push(s1.pop());
21. }
22. }
23. 24. return s2.peek();
25. }
26. 27. // Return whether the queue is empty.
28. public boolean empty() {
29. return s1.isEmpty() && s2.isEmpty();
30. }
31. }