講的很好，有兩種方法

/*
public class ListNode {
    int val;
    ListNode next = null;

    ListNode(int val) {
        this.val = val;
    }
}*/
public class Solution {
    public ListNode ReverseList(ListNode head) {
        if(head==null||head.next==null)
            return head;
        ListNode node=ReverseList(head.next);
        head.next.next=head;
        head.next=null;
        return node;
    }
}