利用ArrayList 实现栈和队列操作

栈

import java.util.ArrayList;

public class MyStack {
    private ArrayList<Integer> stack;

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

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

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

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

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

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

    public static void main(String[] args) {
        MyStack stack = new MyStack();
        stack.push(1);
        stack.push(2);
        stack.push(3);

        System.out.println("Pop: " + stack.pop());
        System.out.println("Peek: " + stack.peek());
        System.out.println("Size: " + stack.size());
        System.out.println("Is Empty: " + stack.isEmpty());
    }
}

队列

import java.util.ArrayList;

public class MyQueue {
    private ArrayList<Integer> queue;

    public MyQueue() {
        queue = new ArrayList<>();
    }

    public void enqueue(int element) {
        queue.add(element);
    }

    public int dequeue() {
        if (isEmpty()) {
            throw new IllegalStateException("Queue is empty");
        }
        return queue.remove(0);
    }

    public int peek() {
        if (isEmpty()) {
            throw new IllegalStateException("Queue is empty");
        }
        return queue.get(0);
    }

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

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

    public static void main(String[] args) {
        MyQueue queue = new MyQueue();

        queue.enqueue(1);
        queue.enqueue(2);
        queue.enqueue(3);

        System.out.println(queue.dequeue()); // Output: 1
        System.out.println(queue.peek()); // Output: 2
        System.out.println(queue.size()); // Output: 2
    }
}