19.2.24 [LeetCode 92] Reverse Linked List II

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

Note: 1 ≤ m ≤ n ≤ length of list.

Example:

Input: 1->2->3->4->5->NULL, m = 2, n = 4
Output: 1->4->3->2->5->NULL

class Solution {
public:
    ListNode* reverseBetween(ListNode* head, int m, int n) {
        ListNode*mnode = head, *nnode = head, *bef = NULL, *aft = NULL, *now = head;
        int cnt = 1;
        while (now) {
            if (cnt == m - 1)
                bef = now;
            else if (cnt == m)
                mnode = now;
            if (cnt == n) {
                nnode = now;
                aft = nnode->next;
                break;
            }
            now = now->next;
            cnt++;
        }
        nnode->next = NULL;
        if(bef)
            bef->next = NULL;
        while (mnode) {
            ListNode*next = mnode->next;
            mnode->next = aft;
            aft = mnode, mnode = next;
        }
        if (bef) {
            bef->next = nnode;
            return head;
        }
        return nnode;
    }
};