225. 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).

题目含义：使用queue来实现stack的功能

 

class MyStack {

    private Queue<Integer> p = new LinkedList<>();

    /** Initialize your data structure here. */
    public MyStack() {

    }

    /** Push element x onto stack. */
    public void push(int x) {
        p.add(x);
        for (int i=1;i<p.size();i++)
        {
            p.add(p.poll());
        }
    }

    /** Removes the element on top of the stack and returns that element. */
    public int pop() {
        return p.poll();
    }

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

    /** Returns whether the stack is empty. */
    public boolean empty() {
        return p.isEmpty();
    }
}