232. 用栈实现队列

class MyQueue {
    Stack<Integer> stackIn;
    Stack<Integer> stackOut;

    public MyQueue() {
        stackIn = new Stack<>();
        stackOut = new Stack<>();
    }

    public void push(int x) {
        stackIn.push(x);
    }

    public int pop() {
        stackChange();
        Integer pop = stackOut.pop();
        return pop;
    }

    public int peek() {
        stackChange();
        Integer peek = stackOut.peek();
        return peek;
    }

    public boolean empty() {
        return stackOut.isEmpty() && stackIn.isEmpty();
    }

    public void stackChange() {
        if(!stackOut.isEmpty()) return; // 自己的代码少了这里
        while (!stackIn.isEmpty()) {
            Integer head = stackIn.pop();
            stackOut.push(head);
        }
    }
}

所以说!!! 思路很重要

225. 用2个队列实现栈

这个写了15分钟写出来

class MyStack {
       Queue<Integer> queueIn;
    Queue<Integer> queueBackUp;

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

    public void push(int x) {
        reBackUp();
        queueIn.offer(x);
    }

    public int pop() {
        reBackUp();
        while (queueIn.size() > 1) {
            Integer front = queueIn.poll();
            queueBackUp.offer(front);
        }
        return queueIn.poll();
    }

    public int top() {
        reBackUp();
        while (queueIn.size() > 1) {
            Integer front = queueIn.poll();
            queueBackUp.offer(front);
        }
        return queueIn.peek();

    }

    public boolean empty() {
        return queueIn.size() == 0 && queueBackUp.size() == 0;
    }

    private void reBackUp() {
        while (queueIn.size() != 0) {
            queueBackUp.offer(queueIn.poll());
        }
        Queue<Integer> tmp = queueIn;
        queueIn = queueBackUp;
        queueBackUp = tmp;
    }
}

其实第二个队列是纯纯的 backup,可以不要

这个写了3分钟写出来

实际上,一个数(假定它叫X)入队之后, 是可以 把X前面的 全部 移到X的后面.

class MyStack {
    Queue<Integer> queue = new LinkedList<>();

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

    public void push(int x) {
        queue.offer(x);
        int move = queue.size() - 1; // 插入一个,前面就是0个,再插入一个前面就是1个
        for (int i = 0; i < move; i++) {
            queue.offer(queue.poll());
        }

    }

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

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

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