/**

* Definition for singly-linked list.

* public class ListNode {

* int val;

* ListNode next;

* ListNode(int x) {

* val = x;

* next = null;

* }

* }

*/

public class Solution {

public ListNode reverseBetween(ListNode head, int m, int n) {

// Start typing your Java solution below

// DO NOT write main() function

if(n-m<=0) return head;

ListNode prev = new ListNode(0);

prev.next = head;

ListNode p = prev;

// reverse from p.next

for(int i=0;i<m-1;i++){

p = p.next;

}

ListNode pnext = p.next;

ListNode qprev = p;

ListNode qnext = p.next.next;

ListNode q = p.next;

for(int i=0;i<n-m;i++){

q.next = qprev;

qprev = q;

q = qnext;

qnext = qnext.next;

}

q.next = qprev;

p.next = q;

pnext.next = qnext;

return prev.next;

}

}