1.写MyStack类，实现栈功能，要求在类中使用ArrayList保存数据。写MyStackTest类对栈功能进行测试。

MyStack类的代码如下：

import java.util.ArrayList;

public class MyStack {
    private ArrayList<Integer> stack;

    public MyStack() {
        stack = new ArrayList<>();
    }

    public void push(int x) {
        stack.add(x);
    }

    public int pop() {
        if (isEmpty()) {
            throw new RuntimeException("Stack is empty");
        }
        return stack.remove(stack.size() - 1);
    }

    public int peek() {
        if (isEmpty()) {
            throw new RuntimeException("Stack is empty");
        }
        return stack.get(stack.size() - 1);
    }

    public boolean isEmpty() {
        return stack.isEmpty();
    }

    public int size() {
        return stack.size();
    }
}

MyStackTest类的代码如下：

public class MyStackTest {
    public static void main(String[] args) {
        MyStack stack = new MyStack();

        // 测试push方法
        stack.push(1);
        stack.push(2);
        stack.push(3);
        System.out.println("Stack size after push: " + stack.size());

        // 测试pop方法
        System.out.println("Pop value: " + stack.pop());
        System.out.println("Pop value: " + stack.pop());
        System.out.println("Stack size after pop: " + stack.size());

        // 测试peek方法
        System.out.println("Peek value: " + stack.peek());
        System.out.println("Stack size after peek: " + stack.size());

        // 测试isEmpty方法
        System.out.println("Stack is empty: " + stack.isEmpty());
        stack.pop();
        System.out.println("Stack is empty: " + stack.isEmpty());
    }
}

输出结果如下：

Stack size after push: 3
Pop value: 3
Pop value: 2
Stack size after pop: 1
Peek value: 1
Stack size after peek: 1
Stack is empty: false
Stack is empty: true