Java--不带头节点的单链表、循环单链表

①不带头节点的单链表

public class Link {
    private class Entry{
        private E value;
        private Entry next;
        public Entry(E value){
            this.value = value;
        }
    }

    private Entry headEntry;
    private Entry tailEntry;
    public Link(){
    }
    //头插
    public void addHead(T value){
        Entry newEntry = new Entry<>(value);
        if(headEntry == null){
            headEntry = newEntry;
            tailEntry = newEntry;
        }else{
            newEntry.next = headEntry;
            headEntry = newEntry;//新头
        }
    }
    //尾插
    public void addTail(T value){
        Entry newEntry = new Entry<>(value);
        if(headEntry == null){
            headEntry = newEntry;
            tailEntry = newEntry;
        }else{
            tailEntry.next = newEntry; 
            tailEntry = newEntry;//更新新尾
        }
    }
    //头删
    public void deleteHead(){
        if(headEntry == null){
            headEntry = null;
            tailEntry = null;
        }
        Entry p = headEntry;
        headEntry = headEntry.next;
        p.value = null;
        p.next = null;
    }
    //尾删
    public void deleteTail() {
        Entry pre = searchPre();
        if (pre == null) {
            return;
        }
        if (headEntry.next == null) {
            headEntry.value = null;
            headEntry = null;
            tailEntry = null;
        } else {
            pre.next = null;
            tailEntry.value = null;
            tailEntry = pre;
        }
    }
    //用来寻找尾巴前驱
     private Entry searchPre(){
        for(Entry p = headEntry;p!=null;p=p.next){
            if(p.next == tailEntry){
                return p;
            }
        }
        return null;
    }
    public void show() {
        for(Entry p = headEntry;p!=null;p=p.next){
            System.out.print(p.value +" ");
        }
        System.out.println();
    }
    //测试
    public static void main(String[] args) {
        Link link=new Link<>();
        link.addHead(1);
        link.addHead(2);
        link.addHead(3);
        link.addTail(4);
        link.addTail(5);
        link.deleteHead();
        link.deleteTail();
        link.show();
    }
}

②不带头节点的循环单链表

public class SingleList {
    private static class Entry{
        private Entry next;
        private E value;
        public Entry(E value){
            this.value = value;
            next = this; //循环
        }
    }
    private Entry headEntry;
    private Entry tailEntry;
    public SingleList(){
    }
    public void addHead(T value){
        Entry newEntry = new Entry<>(value);
        if(headEntry == null){
            headEntry = newEntry;
            tailEntry = newEntry;
        }else{
            //插入的这个元素后一个即原来的头
            newEntry.next = headEntry;
            //更新头
            headEntry = newEntry;
            //尾部下一个指向现在的头
            tailEntry.next = headEntry;
        }
    }
    public void addTail(T value){
        Entry newEntry = new Entry<>(value);
        if(headEntry == null){
            headEntry = newEntry;
            tailEntry = newEntry;
        }else {
            tailEntry.next = newEntry;
            newEntry.next = headEntry;
            tailEntry = newEntry;
        }
    }
    public void deleteHead(){
        if(headEntry == null) //空链表
            return;
        headEntry.value = null;
        if(headEntry.next == headEntry){  //只有一个节点
            headEntry = null;
            tailEntry = null;
        }else {
            tailEntry.next = headEntry.next;
            headEntry = headEntry.next; //更新头
        }
    }
    private Entry searchPre(){
        Entry p = headEntry;
        do{
            if(p.next == tailEntry){
                return p;
            }
            p=p.next;
        }while(p!=headEntry); //此处使用do while比较简单，for循环条件出错容易陷入死循环

        return null;
    }
    public void deleteTail(){
        if(headEntry == null){
            return;
        }
        if(headEntry.next == headEntry){
            headEntry = null;
            tailEntry = null;
        }else {
            Entry pre = searchPre();
            pre.next = tailEntry.next;
            tailEntry.value = null;
            tailEntry = pre;//更新尾
        }
    }

    public void show(){
        if(headEntry == null){
            return;
        }
        Entry p = headEntry;
        do{
            System.out.print(p.value+" ");
            p=p.next;
        }while(p!=headEntry);
    }
    public static void main(String[] args) {
            SingleList singleList = new SingleList<>();
            singleList.addHead(1);
            singleList.addHead(2);
            singleList.addHead(3);
            singleList.addTail(4);
            singleList.addTail(5);
            singleList.deleteHead();
            singleList.deleteTail();
            singleList.show();
        }
}