LeetCode - 92、反转链表 II

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

 /**
  * 列表定义
  * public class ListNode {
  *     int val;
  *     ListNode next;
  *     ListNode(int x) { val = x; }
  * }
  */

解答：

class Solution {
    public ListNode reverseBetween(ListNode head, int m, int n) {
        // Empty list
        if (head == null) {
            return null;
        }

        ListNode cur = head, prev = null;
        while (m > 1) {
            prev = cur;
            cur = cur.next;
            m--;
            n--;
        }

        ListNode con = prev, tail = cur;

        ListNode third = null;
        while (n > 0) {
            third = cur.next;
            cur.next = prev;
            prev = cur;
            cur = third;
            n--;
        }

        if (con != null) {
            con.next = prev;
        } else {
            head = prev;
        }

        tail.next = cur;
        return head;
    }
}