带有“哨兵”的链表：v2

/* 带有哨兵的链表。哨兵所在的位置是真正列表开始前的一位。记住这一点，即可轻松理解哨兵含义，并编写addFirst和addLast方法。*/
// first是否为空影响某些情况的判断，既然这样，找一个哨兵，指向first。就可解决这一问题。这个思路很好。

public class SLList_1{
    public static class IntNode{
        public int item;
        public IntNode next;

        public IntNode(int i, IntNode n){
            item = i;
            next = n;
        }
    }
    public IntNode sentry;
    public int size;

    public SLList_1(){
        sentry = new IntNode(1, null);
        size = 0;
    }
    public SLList_1(int x){
        sentry = new IntNode(1, null);
        sentry.next = new IntNode(x, null);
        size = 1;
    }
    public void addFirst(int x){
        sentry.next = new IntNode(x, sentry.next);
        size += 1;
    }
    public void addLast(int x){
        // 不论此时真正的链表是否为空，都不影响。
        IntNode p = sentry;
        if(p.next != null){
            p = p.next;
        }
        p.next = new IntNode(x, null);
        size += 1;
    }

    public int getSize() {
        return size;
    }

    public static void main(String[] args) {
        SLList_1 L = new SLList_1();
        L.addFirst(5);
        L.addLast(10);
        System.out.print(L.getSize());
    }
}