LeetCode225. Implement Stack using Queues

方法1：只用一个队列
出栈直接出队，但是入栈前先直接入队，然后出队n-1个元素并入队，这样刚进队的元素就在队头了。
上代码：

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

方法二：采用两个队helpQ和pollQ
入栈时先将pollQ的元素出队并存入helpQ中，然后新元素进pollQ，再将helpQ中的所有元素出队依次进pollQ
代码：

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

如有错误，请指正！

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