/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode reverseBetween(ListNode head, int m, int n) {
        ListNode dummy = new ListNode(0);
        dummy.next = head;
        ListNode pre = dummy;
        ListNode cur = dummy.next;
        int counter = n - m;
        
        dummy.next = head;
        
        for (int i = 1; i < m; i++) {
            cur = cur.next;
            pre = pre.next;
        }
        // 1  -> 2  -> 3  ->    4     ->  5  ->  NULL, m = 2, n = 4
        // pre  cur  after  after.next
        // 1  -> 3 ->  2   ->  4  ->    5    ->  NULL, m = 2, n = 4
        // pre         cur  after   after.next
        // 1  -> 4 ->  3 ->  2  ->  5    ->    NULL, m = 2, n = 4
        // pre              cur   after     after.next
        for (int i = 0; i < n -m; i++) {
            ListNode after = cur.next;
            // 1   -> 2  ->  4
            // pre   cur   
            cur.next = after.next;
            //  3 -> 2
            after.next = pre.next;
            // 1 -> 3
            pre.next = after;
        }

        return dummy.next;
    }
}

class Solution:
    def reverseBetween(self, head, m, n):
        """
        :type head: ListNode
        :type m: int
        :type n: int
        :rtype: ListNode
        """
        dummy = ListNode(None)
        dummy.next = head
        pre = dummy
        curr = dummy.next
        for i in range(1, m):
            curr = curr.next
            pre = pre.next
        for i in range(n-m):
            temp = curr.next
            curr.next = temp.next
            temp.next = pre.next
            pre.next = temp
        return dummy.next