/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode reverseBetween(ListNode head, int left, int right) {
        ListNode dummy = new ListNode(0, head);
        ListNode p0 = dummy;
        for(int i = 0; i < left - 1; i ++ ) p0 = p0.next;

        ListNode pre = null, cur = p0.next;
        for(int i = 0; i < right - left + 1; i ++ ){
            ListNode nxt = cur.next;
            cur.next = pre;
            pre = cur;
            cur = nxt;
        }

        p0.next.next = cur;
        p0.next = pre;

        return dummy.next;
    }
}