Solution.java

/**
 * 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, head);
        ListNode pre = dummy, cur = dummy;
        while (cur.next != null) {
            for (int i = 0; i < k && cur != null; ++i) {
                cur = cur.next;
            }
            if (cur == null) {
                return dummy.next;
            }
            ListNode t = cur.next;
            cur.next = null;
            ListNode start = pre.next;
            pre.next = reverseList(start);
            start.next = t;
            pre = start;
            cur = pre;
        }
        return dummy.next;
    }

    private ListNode reverseList(ListNode head) {
        ListNode pre = null, p = head;
        while (p != null) {
            ListNode q = p.next;
            p.next = pre;
            pre = p;
            p = q;
        }
        return pre;
    }
}