206. 反转链表

pre=NULL，now=head，while(now!=NULL)

  1. /**
  2.  * Definition for singly-linked list.
  3.  * struct ListNode {
  4.  *     int val;
  5.  *     ListNode *next;
  6.  *     ListNode() : val(0), next(nullptr) {}
  7.  *     ListNode(int x) : val(x), next(nullptr) {}
  8.  *     ListNode(int x, ListNode *next) : val(x), next(next) {}
  9.  * };
  10.  */
  11. class Solution {
  12. public:
  13.     ListNode* reverseList(ListNode* head) {
  14.         ListNode* pre = nullptr;
  15.         ListNode* now = head;
  16.         while(now!=nullptr){
  17.             ListNode* next = now->next;
  18.             now->next = pre;
  19.             pre = now;
  20.             now = next;
  21.         }
  22.         return pre;
  23.     }
  24. };