算法系列------反转链表2

反转从位置 m 到 n 的链表。请使用一趟扫描完成反转。

说明:
1 ≤ m ≤ n ≤ 链表长度。

示例:

输入: 1->2->3->4->5->NULL, m = 2, n = 4
输出: 1->4->3->2->5->NULL

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode reverseBetween(ListNode head, int m, int n) {
        if(head == null) 
            return head;
        ListNode dummy = new ListNode(0);
        dummy.next = head;
        ListNode prev = dummy;
        for(int i=1; i<m; i++){
            prev = prev.next;
        }
        head = prev.next;//2
        for(int i=m; i<n; i++){
            ListNode nex = head.next;
            head.next = nex.next;
            nex.next = prev.next;
            prev.next = nex;
        }
        return dummy.next;
    }
}