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 };