Leetcode 92. Reverse Linked List II

/**
* Definition for singly-linked list.
* public class ListNode {
*     int val;
*     ListNode next;
*     ListNode(int x) { val = x; }
* }
*/
public class Solution {
public ListNode reverseBetween(ListNode head, int m, int n) {
if(head==null) return head;
ListNode mNode = new ListNode(0);
ListNode p = mNode;
mNode.next = head;
for(int i=0;i<=m-2;i++){
mNode = mNode.next;
}
ListNode mNodeNext = mNode.next;
for(int i=m;i<n;i++){
// insert node between m and n before m
// change pointer after m
ListNode q = mNodeNext.next;
mNodeNext.next = q.next;
// change pointer before m
q.next = mNode.next;
mNode.next = q;
}

return p.next;
}
}