class Solution {
    public ListNode cur;
    public ListNode reverseKGroup(ListNode head, int k) {
        cur = head;
        ListNode pre, ans;
        ans = null;
        pre = null;
        while(cur!=null){
            head = cur;
            ListNode t = reverse(cur, k);
            if(pre!=null) pre.next = t;
            else ans = t==null? cur: t;
            if(ans==null) return head;
            if(t==null) pre.next = head;
            pre = head; 
        }
        return ans;
    }

    public ListNode reverse(ListNode head, int k){
        if(head==null||(head.next==null&&k>1)) return cur = null;
        if(k==1){
            cur = head.next;
            return head;
        } 
        ListNode t = reverse(head.next, k-1);
        if(t!=null){
            head.next.next = head;
            head.next = null;
        }
        return t;
    }
}

正常思路 
记住pre start end 三点就可以了

class Solution {
    public ListNode reverseKGroup(ListNode head, int k) {
        ListNode dumy = new ListNode(0);
        dumy.next = head;
        ListNode pre, end, start;
        pre = dumy;
        end = dumy;
        while(end!=null){
            for(int i=0; i<k&&end!=null; i++) end = end.next;
            if(end==null) break;
            ListNode next = end.next;
            start = pre.next;
            reverse(start, k);
            pre.next = end;
            pre = start;
            pre.next = next;
            end = pre;
        }
        return dumy.next;

    }
    public void reverse(ListNode start, int k){
        ListNode pre, cur, next;
        pre = null;
        cur = start;
        for(int i=0; i<k; i++){
            next = cur.next;
            cur.next = pre;
            pre = cur;
            cur = next;
        }
    }
}