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).

 class Queue {
     stack<int> s1;
     stack<int> s2;
 public:
     // Push element x to the back of queue.
     void push(int x) {
         while(!s1.empty()){
             s2.push(s1.top());
             s1.pop();
         }
         s1.push(x);
         while(!s2.empty()){
             s1.push(s2.top());
             s2.pop();
         }
     }

     // Removes the element from in front of queue.
     void pop(void) {
         s1.pop();
     }

     // Get the front element.
     int peek(void) {
         return s1.top();
     }

     // Return whether the queue is empty.
     bool empty(void) {
         return s1.empty();
     }
 };