【LeetCode】206 - Reverse Linked List

Reverse a singly linked list.

Hint:A linked list can be reversed either iteratively or recursively. Could you implement both?

Solution 1:iteration

 1 /**
 2  * Definition for singly-linked list.
 3  * struct ListNode {
 4  *     int val;
 5  *     ListNode *next;
 6  *     ListNode(int x) : val(x), next(NULL) {}
 7  * };
 8  */
 9 class Solution {
10 public:
11     ListNode* reverseList(ListNode* head) {        //runtime:8ms
12         ListNode *p1,*p2,*p3;
13         if(head==NULL||head->next==NULL)return head;
14         p1=head,p2=p1->next;
15         while(p2){
16             p3=p2->next;
17             p2->next=p1;
18             p1=p2;
19             p2=p3;
20         }
21         head->next=NULL;
22         head=p1;
23         return head;
24     }
25 };

Solution 2:recursion

 1 class Solution {
 2 public:
 3     ListNode* reverseList(ListNode* head) {     //runtime:8ms
 4         if(head==NULL||head->next==NULL)return head;
 5
 6         ListNode* p = head->next;
 7         ListNode* n = reverseList(p);
 8
 9         head->next = NULL;
10         p->next = head;
11         return n;
12     }
13 };