[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.

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* reverseBetween(ListNode* head, int m, int n) {
        if (head == NULL) return NULL;
        
        ListNode** pre = &head;
        
        for (int i = 0; i < m - 1; i++)
            pre = &((*pre)->next);
        
        n -= m;
        ListNode* start = *pre;
        while (n--) {
            ListNode* p = start->next;
            start->next = p->next;
            p->next = *pre;
            (*pre) = p;
        }
        
        return head;
    }
};