单链表的增删改查（带头节点）

带头节点 的单链表：在初始化时产生一个headEntry结点，其value域和next域均为空。其链表结构如下：

注意：带头节点和不带头节点的单链表区别就在headEntry

带头节点的单链表我们需要从headEntry的下个节点开始进行各种操作，而不带头节点的从headEntry开始操作，操作方法大同小异，适当分析，搞清楚起始点即可

• Entry类封装节点属性，包括value域及next域
• 代码实现如下：

public class Entry<E extends Comparable<E>> {
    private E value;
    private Entry next;
    public Entry(){
    }
    public Entry(E value){
        this.value=value;
    }
    public void setValue(E value) {
        this.value = value;
    }
    public void setNext(Entry next) {
        this.next = next;
    }
    public E getValue() {
        return value;
    }
    public Entry getNext() {
        return next;
    }
}

• Link类封装头插尾插头删尾删等操作
• 代码实现如下：

public class Link<T extends Comparable<T>> {
    private Entry<T> headEntry;
    private Entry<T> tailEntry;
    public Link(){         //初始化headEntry节点
        headEntry=new Entry<>();
    }
    
    //头插
    public void addHead(T value){
        Entry<T> newEntry=new Entry<>(value);
        if(headEntry.getNext()==null){
            headEntry.setNext(newEntry);
            tailEntry=newEntry;
        }
        newEntry.setNext(headEntry.getNext());
        headEntry.setNext(newEntry);//更新头节点
    }
    
    //尾插
    public void addTail(T value){
        Entry<T> newEntry=new Entry<>(value);
        if(headEntry.getNext()==null){
            headEntry.setNext(newEntry);
            tailEntry=newEntry;
        }
        tailEntry.setNext(newEntry);
        tailEntry=newEntry;//更新尾节点
    }
    
    //头删
    public void deleteHead(){
        if(headEntry.getNext()==null){
            return;
        }
        Entry p=headEntry.getNext();
        p.setValue(null);//防止内存泄漏
        headEntry.setNext(headEntry.getNext().getNext());
    }
    
    //尾删
    public void deleteTail(){
        if(headEntry.getNext()==null){
            return;
        }
        Entry<T> p=headEntry.getNext();
        for(;p.getNext().getNext().getNext()!=null;p=p.getNext()){
            ;   //找尾部前驱p
        }
        p.getNext().setNext(null);
        tailEntry=p.getNext();
    }
    
    //删除指定节点
    public void deleteValue(T value){
        if(headEntry.getNext()==null){
            return;
        }
        if(headEntry.getNext().getValue().compareTo(value)==0){
            deleteHead();
            if(headEntry==tailEntry) {
                tailEntry.setNext(null);
            }
            return;
        }
        if(tailEntry.getValue().compareTo(value)==0){
            deleteTail();
            if(headEntry==tailEntry) {
                tailEntry.setNext(null);
            }
            return;
        }
        for(Entry<T> p=headEntry.getNext();p.getNext().getNext()!=null;p=p.getNext()){
            if(p.getNext().getValue().compareTo(value)==0){
                p.getNext().setValue(null);
                p.setNext(p.getNext().getNext());
                break;
            }
        }
    }
}