反转链表

public class ListNode{
    int val;
    ListNode next = null;
    
    ListNode(int val){
        this.val = val;
    }
}

public class Solution {
    public ListNode ReverseList(ListNode head) {
        ListNode node = head；  //node为当前操作节点
        ListNode rhead = head;  //rhead为反转后的链表头
        ListNode b = null;      //node的前一节点
        while(node!=null){
            ListNode p = node.next; //要把节点node的下一个节点保存起来
            if(p==null){
                rhead = node;
            }
            node.next = b;
            b = node;
            node = p;
        }
        return rhead;
    }
}