单链表反转

• 都是一个思路，在遍历的过程中改变指针。

递归卡住了，我想的是利用return来完成指针的修改，但这样拿不到整个反转链表的头
最后看的别人的，return专门用来传递反转链表的头
first.next = null；如果忽略，最后两个节点会不断循环

两指针

    public static  Node reverse1(Node first) {
        if(first == null)
            return null;
        Node prev = null;
        Node next = first;
        for (Node node = first; node != null; node = next) {
            next = node.next;
            node.next = prev;
            prev = node;
        }
        return prev;
    }
//    反转链表 递归
    public static  Node reverse2(Node first)  {
        if(first == null||first.next == null)
            return first;
        else  {
//            head代表整个队列的头，first是每次递归的头
            Node head = reverse2(first.next);
            first.next.next = first;
            first.next = null;
            return head;
        }
    }

递归

//    反转链表 递归
    public static  Node reverse2(Node first)  {
        if(first == null||first.next == null)
            return first;
        else  {
//            head代表整个队列的头，first是每次递归的头
            Node head = reverse2(first.next);
            first.next.next = first;
            first.next = null;
            return head;
        }
    }

递归优化

//    反转链表 递归优化
    public static  Node reverse3(Node first) {
        if(first == null)
            return null;
        Stack> stack = new Stack<>();
        for (Node node = first;node!=null;node = node.next){
            stack.push(node);
        }
        first = stack.pop();
        Node prev;
        while(!stack.isEmpty()){
            prev = stack.pop();
            prev.next.next = prev;
            System.out.println("item"+prev.item);
            prev.next = null;
        }
        return first;
    }

Node类

public class Node {
    Node next;
    Node prev;

    T item;

    public Node() {
    }

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

    public Node(Node prev,T item,Node next) {

        this.prev = prev;
        this.item = item;
        this.next = next;
    }
// 最后返回的是item的toString
    @Override
    public String toString() {
        return item.toString();
    }
}