[LeetCode] Reverse Linked List II

Reverse a linked list from position m to n. Do it in-place and in one-pass.

For example:
Given 1->2->3->4->5->NULL, m = 2 and n = 4,

return 1->4->3->2->5->NULL.

Note:
Given m, n satisfy the following condition:
1 ≤ m ≤ n ≤ length of list.

 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* reverseBetween(ListNode* head, int m, int n) {
12         if (head == NULL) return NULL;
13         
14         ListNode** pre = &head;
15         
16         for (int i = 0; i < m - 1; i++)
17             pre = &((*pre)->next);
18         
19         n -= m;
20         ListNode* start = *pre;
21         while (n--) {
22             ListNode* p = start->next;
23             start->next = p->next;
24             p->next = *pre;
25             (*pre) = p;
26         }
27         
28         return head;
29     }
30 };