225_ImplementStackusingQueue.java

// 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).



class MyStack {
    // Push element x onto stack.
    Queue<Integer> q = new LinkedList<>();
    Queue<Integer> temp = new LinkedList<>();
    public void push(int x) {
        while(!q.isEmpty()) {
            temp.offer(q.poll());
        }
        q.offer(x);
        while(!temp.isEmpty()) {
            q.offer(temp.poll());
        }
    }

    // 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.isEmpty();
    }
}