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).

 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 };