Implementing Stack Using Queue

Question:

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 emptyoperations 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).

Thoughts:

One Simple Solution is Using two queues to build a stack, we initiate two queues which are q1 and q2(q2 is a temprory queue), every time the "Stack" is about to push, we add all the elements in q1 to q2, and push the new element into q1 to make sure the new element comes out the first.

However, there's one method that can build this stack class with only one Queue with O(n) time, the whole point is to remain the Queue as it is, but every time we perform Pop() and Top() function, we remove all of the elements in the queue except the very last one at the bottom originally, and push all these removed elements back to it's own Queue after the "original bottom" element, in this way, we keep the Queue structure as it is and we fetch the Bottom element in the queue, which help us to perform the Pop() adn Top() function.

Code:

class MyStack {

    // Push element x onto stack.

    Queue<Integer> q = new LinkedList<Integer> ();

    public void push(int x) {

        q.add(x);

    }



    // Removes the element on top of the stack.

    public void pop() {

        if(!q.isEmpty()){

        int length = q.size();

        for(int i = 1 ; i<length; i++){

         q.add(q.remove()) ;  

        }

        q.remove();

    }

    else return;

    }



    // Get the top element.

    public int top() {

        if(!q.isEmpty()){

          int size = q.size();

        for(int i = 1; i < size; i++)

            q.add(q.remove());

        int ret = q.remove();

        q.add(ret);

        return ret;

        }

        else return -1;

    }



    // Return whether the stack is empty.

    public boolean empty() {

        return q.isEmpty();

    }

}