232. Implement Queue using Stacks Java Solutions

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).

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