92. 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.

/**
 * 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) {
        ListNode dummy = new ListNode(-1);
        dummy.next = head;
        ListNode p = dummy;
        int k = 0;
        while( ++k < m){
            if(p != null) p = p.next;
        }
        //TODO:
        ListNode tail = p.next;
        while(++k <= n){
            ListNode tmp = p.next;

            p.next = tail.next;
            tail.next = tail.next.next;
            p.next.next = tmp;


        }

        return dummy.next;


    }
}