[Leetcode] Reverse Linked List II

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

For example:
Given 1->2->3->4->5->NULL, m = 2 and n = 4,

return 1->4->3->2->5->NULL.

Note:
Given m, n satisfy the following condition:
1 ≤ m ≤ n ≤ length of list.

 

Solution:

/**
 * 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) {
        if(m==n)
            return head;
        ListNode dummy=new ListNode(-1);
        dummy.next=head;
        ListNode pointer=dummy;
        int cnt=m-1;
        while(cnt!=0){
            pointer=pointer.next;
            cnt--;
        }
        ListNode l1,l2,l3;
        l1=pointer.next;
        ListNode last=l1;
        l2=l1.next;
        l3=l2.next;
        cnt=n-m-1;
        while(cnt!=0){
            l2.next=l1;
            
            l1=l2;
            l2=l3;
            l3=l3.next;
            cnt--;
        }
        l2.next=l1;
        last.next=l3;
        pointer.next=l2;
        return dummy.next;
    }
}