下压堆栈（链表实现）

public class Node<Item> {
    Item item;
    Node<Item> next;
    public Node() {
        next = null;
    }
}

堆栈的实现

public class Stack<Item> {
    
    private Node<Item> first;
    private int N;
    public boolean isEmpty(){
        return first == null;
    }
    public int size(){
        return N;
    }
    public void push (Item item){
        Node<Item> oldfirst = first;
        first = new Node<>();
        first.item = item;
        first.next = oldfirst;
        N++;
    }
    
    public Item pop() {
        if (this.isEmpty()) {
            return null;
        }
        Item item = first.item;
        first = first.next;
        N--;
        return item;
    }
    
    public static void main(String[] args) {
        
        Stack<String> stack = new Stack<>();
        stack.push("to");
        stack.push("be");
        stack.push("or");
        System.out.println(stack.pop());
        System.out.println(stack.pop());
        System.out.println(stack.pop());
        System.out.println(stack.pop());
    }

}
/////////////
or
be
to
null