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).

补充知识：linkedList的poll方法：此方法返回此列表的头元素，或null，如果此列表为空

http://blog.csdn.net/gongchuangsu/article/details/51527042

peek():获取但不移除此列表的头

poll():获取并移除此列表的头

Only push is O(n), others are O(1). 

public class MyStack {    /** Initialize your data structure here. */    public MyStack() {            }        Queue<Integer> q = new LinkedList<Integer>();        /** Push element x onto stack. */    public void push(int x) {        q.add(x); //linklist的add是在list的最后add        for(int i=1;i<q.size();i++){            q.add(q.poll());//linklist的poll是在list开头检索移除        }    }        /** Removes the element on top of the stack and returns that element. */    public int pop() {        return q.poll();    }        /** Get the top element. */    public int top() {        return q.peek();    }        /** Returns whether the stack is empty. */    public boolean empty() {        return q.isEmpty();    }}/** * Your MyStack object will be instantiated and called as such: * MyStack obj = new MyStack(); * obj.push(x); * int param_2 = obj.pop(); * int param_3 = obj.top(); * boolean param_4 = obj.empty(); */