题解 | #反转链表#

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

    ListNode(int val) {
        this.val = val;
    }
}*/
//新建链表
  class Solution {
     public ListNode ReverseList(ListNode head) {
         if(head==null){
             return null;
         }
         ListNode headA=new ListNode(head.val);
         headA.next = null;
         head = head.next;
         while(head!=null){
             ListNode p = new ListNode(head.val);
             p.next = headA;
             headA = p;
             head = head.next;
         }
         return headA;
     }
 }

//该方***改变head的结构，作为传递参数为什么会改变呢？？？？
 class Solution {
    public ListNode ReverseList(ListNode head) {
        if(head==null){
            return null;
        }
        ListNode curr = head,pre = null,next;
        while(curr!=null){
            next = curr.next;	//next指向未反转链表的第一个节点的下一节点，用于保存数据；
            curr.next = pre;	//将当前节点的下一节点指向已反转链表的头节点
            pre = curr;			//将pre指针向当前节点
            curr = next;		//将curr指针指向next节点，也就是pre，curr指针向后移动
        }
        return pre;				//最后返回pre指针，该指针指向头节点
    }
}