Reverse Nodes in k-Group.java

class Solution {
    public ListNode reverseKGroup(ListNode head, int k) {
       if(head==null||k==1)
            return head;

        ListNode f = new ListNode(0);
        f.next = head;
        ListNode pre = f;
        int i=0;

        ListNode p = head;
        while(p!=null){
            i++;
            if(i%k==0){
                pre = reverse(pre, p.next);
                p = pre.next;
            }else{
                p = p.next; 
            }
        }

        return f.next; 
    }
    public ListNode reverse(ListNode pre, ListNode next){
        ListNode last = pre.next;
        ListNode curr = last.next;

        while(curr != next){
            last.next = curr.next;
            curr.next = pre.next;
            pre.next = curr;
            curr = last.next;
        }

        return last; 
    }
}