Implement Stack using Queues

Implement the following operations of a stack using queues.

• push(x) -- Push element x onto stack.
• pop() -- Removes the element on top of the stack.
• top() -- Get the top element.
• empty() -- Return whether the stack is empty.

Notes:

• You must use only standard operations of a queue -- which means only push to back, peek/pop from front, size, and is empty operations are valid.
• Depending on your language, queue may not be supported natively. You may simulate a queue by using a list or deque (double-ended queue), as long as you use only standard operations of a queue.
• You may assume that all operations are valid (for example, no pop or top operations will be called on an empty stack).

 

Update (2015-06-11):
The class name of the Java function had been updated to MyStack instead of Stack.

 1 class Stack {

 2 private:

 3     queue <int> q_1;

 4     queue <int> q_2;

 5 public:

 6     // Push element x onto stack.

 7     void push(int x) {

 8         if(q_1.empty())

 9           q_1.push(x);

10         else

11         {

12             while(!q_1.empty())

13             {

14                 q_2.push(q_1.front());

15                 q_1.pop();

16             }

17             q_1.push(x);

18             while(!q_2.empty())

19             {

20                 q_1.push(q_2.front());

21                 q_2.pop();

22             }

23         }

24     }

25 

26     // Removes the element on top of the stack.

27     void pop() {

28         q_1.pop();

29     }

30 

31     // Get the top element.

32     int top() {

33         return q_1.front();

34     }

35 

36     // Return whether the stack is empty.

37     bool empty() {

38         return q_1.empty();

39     }

40 };