225 Implement Stack using Queues – Easy

Problem:

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).

Thoughts:

In order to use a queue to implement stack, we can do it by making push or making pop operation expensive. Expensive means it requires more running time. But consider that we also have function of top, so I choose to make push expensive in the solution below.

Actually, it should depend on how often are those functions called in use case.

Solutions:

class MyStack {
    Queue<Integer> q = new LinkedList<Integer>();
    // Push element x onto stack.
    public void push(int x) {
        Queue<Integer> newq = new LinkedList<Integer>();
        newq.add(x);
        while (q.size() > 0) {
            newq.add(q.poll());
        }
        q = newq;
    }

    // Removes the element on top of the stack.
    public void pop() {
        q.poll();
    }

    // Get the top element.
    public int top() {
        return q.peek();
    }

    // Return whether the stack is empty.
    public boolean empty() {
        return q.size() == 0;
    }
}