/**
 * Definition for singly-linked list.
 * type ListNode struct {
 *     Val int
 *     Next *ListNode
 * }
 */
func reverseKGroup(head *ListNode, k int) *ListNode {
    if head == nil || head.Next == nil || k <= 1 {
        return head
    }

    var prev *ListNode
    current := head
    for i:=0;i<k;i++ {
        prev = current
        if current == nil {
            return head
        }
        current = current.Next
    }
    prev.Next = nil
    res := reverseListNode(head)
    head.Next = reverseKGroup(current, k)

    return res
}

func reverseListNode (head *ListNode) (res *ListNode) {
    var prev *ListNode
    for head != nil {
        next := head.Next
        if res == nil {
            res = head
            prev = head
        } else {
            temp := res
            prev.Next = head.Next
            res = head
            res.Next = temp
        }
        head = next
    }

    return
}