public class Stack {
    private Node last;
    private int size;

    public Stack() {
        last = null;
        size = 0;
    }
    public void push(int item) {//往栈中添加元素
        Node newNode;
        if (size == 0) {
            newNode = new Node(null, null, item);
        } else {
            newNode = new Node(last, null, item);
            last.next = newNode;
        }
        last = newNode;
        size++;
    }

    public int pop() throws Exception {//栈顶元素出栈
        if (size == 0) {
            throw new Exception("StackEmpty");
        } else {
            int res = last.item;
            if (size == 1) {
                last = null;
            } else {
                last = last.perv;
                last.next = null;
            }
            size--;
            return res;
        }
    }

    public int peek() {//查看栈顶元素
        return last.item;
    }

    private Node jump(int position) throws Exception {//通过用户给予的位置信息，移动至指定位置
        if (position > 0 && position <= size) {
            Node nowNode = last;
            int frequency = size - position;
            while (frequency != 0) {
                nowNode = nowNode.perv;
                frequency--;
            }
            return nowNode;
        } else {
            throw new Exception("error...");
        }
    }

    private static class Node {
        private final Node perv;
        private Node next;
        private final int item;

        public Node(Node perv, Node next, int item) {
            this.perv = perv;
            this.next = next;
            this.item = item;
        }
    }
}