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).



1 class Queue {
 2     stack<int> s1;
 3     stack<int> s2;
 4 public:
 5     // Push element x to the back of queue.
 6     void push(int x) {
 7         while(!s1.empty()){
 8             s2.push(s1.top());
 9             s1.pop();
10         }
11         s1.push(x);
12         while(!s2.empty()){
13             s1.push(s2.top());
14             s2.pop();
15         }
16     }
17 
18     // Removes the element from in front of queue.
19     void pop(void) {
20         s1.pop();
21     }
22 
23     // Get the front element.
24     int peek(void) {
25         return s1.top();
26     }
27 
28     // Return whether the queue is empty.
29     bool empty(void) {
30         return s1.empty();
31     }
32 };