25. K 个一组翻转链表

给你链表的头节点 head ，每 k 个节点一组进行翻转，请你返回修改后的链表。

k 是一个正整数，它的值小于或等于链表的长度。如果节点总数不是 k 的整数倍，那么请将最后剩余的节点保持原有顺序。

你不能只是单纯的改变节点内部的值，而是需要实际进行节点交换。

 

示例 1：

输入：head = [1,2,3,4,5], k = 2
输出：[2,1,4,3,5]

示例 2：

输入：head = [1,2,3,4,5], k = 3
输出：[3,2,1,4,5]

 

提示：

• 链表中的节点数目为 n
• 1 <= k <= n <= 5000
• 0 <= Node.val <= 1000

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode reverseKGroup(ListNode head, int k) {
    ListNode dummy = new ListNode(0);
    dummy.next = head;

    ListNode pre = dummy;
    ListNode end = dummy;

    while (end.next != null) {
        for (int i = 0; i < k && end != null; i++) end = end.next;
        if (end == null) break;
        ListNode start = pre.next;
        ListNode next = end.next;
        end.next = null;
        pre.next = reverse(start);
        start.next = next;
        pre = start;

        end = pre;
    }
    return dummy.next;
}

private ListNode reverse(ListNode head) {
    ListNode pre = null;
    ListNode curr = head;
    while (curr != null) {
        ListNode next = curr.next;
        curr.next = pre;
        pre = curr;
        curr = next;
    }
    return pre;
}
}