225. Implement Stack using Queues(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).

思路：用两个队列q1，q2实现一个栈。push时把新元素添加到q1的队尾。pop时把q1中除最后一个元素外逐个添加到q2中，然后pop掉q1中的最后一个元素，然后交换q1和q2，以保证添加元素时始终向q1中添加。

public class MyStack {    private Queue<Integer> q1;    private Queue<Integer> q2;    /** Initialize your data structure here. */    public MyStack() {        q1=new LinkedList<>();        q2=new LinkedList<>();            }        /** Push element x onto stack. */    public void push(int x) {        q1.offer(x);            }        /** Removes the element on top of the stack and returns that element. */    public int pop() {        while(q1.size()>1){            q2.offer(q1.poll());        }        int pop=q1.poll();         //while(q2.isEmpty()){        //    q1.add(q2.remove());}        Queue<Integer> tmp=q1;        q1=q2;        q2=tmp;        return pop;    }        /** Get the top element. */    public int top() {        while(q1.size()>1){            q2.offer(q1.poll());        }        int top=q1.peek();        q2.offer(q1.poll());                Queue<Integer> tmp=q1;        q1=q2;        q2=tmp;        return top;    }        /** Returns whether the stack is empty. */    public boolean empty() {        return q1.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(); */