Leetcode 92. Reverse Linked List II

思路：添加头节点，反转链表某个连续部分，206. Reverse Linked List是本题的特殊情况，也可以像本题一样做，具体见Leetcode 206. Reverse Linked List。

/**
 * 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 pre = dummy;
        for(int i = 0; i < m - 1; i++) pre = pre.next;//移动m-1次
        ListNode start = pre.next;
        ListNode then = start.next;
        for(int i = 0; i < n - m; i++) {//需要将start后面的n-m个点陆续反转
            start.next = then.next;//start的后面指向then的后面
            then.next = pre.next;//then的后面指向pre的后面，相当于将then插入pre和pre.next之间
            pre.next = then;//pre的后面是then
            then = start.next;//完成一个元素的反转后，then指向下一个准备被反转（插入pre和pre.next之间）的节点
        }
        return dummy.next;
    }
}

Next challenges: Swap Nodes in Pairs Remove Duplicates from Sorted List II Delete Node in a Linked List

Next challenges: Reverse Nodes in k-Group Linked List Cycle II Intersection of Two Linked Lists