代码随想录算法训练营第十天| 232. 用栈实现队列 225. 用队列实现栈

232. 用栈实现队列
https://leetcode.cn/problems/implement-queue-using-stacks/description/

class MyQueue {

    Stack<Integer> stackIn;
    Stack<Integer> stackOut;
    public MyQueue() {
        stackIn = new Stack<>();
        stackOut = new Stack<>();
    }
    public void push(int x){
        stackIn.add(x);
    }
    public int pop(){
        dumpstackIn();
        return stackOut.pop();
    }
    public int peek(){
        dumpstackIn();
        return stackOut.peek();
    }
    public boolean empty(){
        return stackOut.isEmpty() && stackIn.isEmpty();
    }
    public void dumpstackIn(){
        if (stackOut.isEmpty()){
            while (!stackIn.isEmpty()){
                stackOut.push(stackIn.pop());
            }
        }
    }
}

总结：两个栈，栈顶放一起，栈顶向两边，重点就是out栈空了的时候从in栈拿元素放out栈里
225. 用队列实现栈
https://leetcode.cn/problems/implement-stack-using-queues/description/

Queue<Integer> queue;

    public MyStack() {
        queue = new LinkedList<>();
    }

    //每 offer 一个数（A）进来，都重新排列，把这个数（A）放到队列的队首
    public void push(int x) {
        queue.offer(x);
        int size = queue.size();
        //移动除了 A 的其它数
        while (size-- > 1)
            queue.offer(queue.poll());
    }

    public int pop() {
        return queue.poll();
    }

    public int top() {
        return queue.peek();
    }

    public boolean empty() {
        return queue.isEmpty();
    }

总结：每次插入元素之后都让他去队首（把队列中的元素重新排列）