1. 栈基础

栈的特征：

• 存取受限的线性表
• 后进先出

栈的操作：

• push()
• pop()
• peek()
• empty()

2.数组实现栈 

限制数组的存取规则，实现后进先出。注意数组边界的处理

public class Stack1<T> {
    private Object[] stack;
    private int top;
    //初始容量
    Stack1(){
        stack = new Object[10];
    }
    public boolean isEmpty(){
        return top == 0;
    }
    public T peek(){
        if (top == 0){
            throw new EmptyStackException();
        }
        return (T) stack[top-1];
    }

    public void push(T oj){
        expandSize(top + 1);
        stack[top] = oj;
        top++;
    }

    public T pop(){
        T peek = peek();
        stack[top - 1] = null;
        top--;
        return peek;
    }

    public void expandSize(int size){
        int len = stack.length;
        if(size > len){
            size = len * 2;
            stack = Arrays.copyOf(this.stack, size);
        }
    }

    public static void main(String[] args) {
        Stack1<String> stack = new Stack1<>();
//        System.out.println(stack.peek());
        System.out.println(stack.isEmpty());
        stack.push("java");
        stack.push("is");
        stack.push("beautiful");
        stack.push("language");
        System.out.println(stack.pop());
        System.out.println(stack.isEmpty());
        System.out.println(stack.peek());
    }
}

3.链表实现栈

使用头插法，达到后进先出效果

class ListStack<T> {
    //定义链表
    class Node<T> {
        public T t;
        public Node next;
    }

    public Node<T> head;

    //构造函数初始化头指针
    ListStack() {
        head = null;
    }

    //入栈
    public void push(T t) {
        if (t == null) {
            throw new NullPointerException("参数不能为空");
        }
        if (head == null) {
            head = new Node<T>();
            head.t = t;
            head.next = null;
        } else {
            Node<T> temp = head;
            head = new Node<>();
            head.t = t;
            head.next = temp;
        }
    }

    //出栈
    public T pop() {
        if (head == null) {
            return null;
        }
        T t = head.t;
        head = head.next;
        return t;
    }

    //取栈顶元素
    public T peek() {
        if (head == null) {
            return null;
        }
        T t = head.t;
        return t;
    }


    //栈空
    public boolean isEmpty() {
        if (head == null)
            return true;
        else
            return false;
    }

    public static void main(String[] args) {
        ListStack stack = new ListStack();
        System.out.println(stack.isEmpty());
        stack.push("Java");
        stack.push("is");
        stack.push("beautiful");
        System.out.println(stack.peek());
        System.out.println(stack.pop());
        System.out.println(stack.isEmpty());
    }
}