public class ReverseNodesInKGroup {

public ListNode reverseKGroup(ListNode head, int k) {

int n = 0;

for (ListNode node = head; node != null; node = node.next, n++);

ListNode dummy = new ListNode(0), cur = dummy, node = head;

for ( ; n >= k; n -= k) {

ListNode tail = node, next;

for (int i = 0; i < k; i++) {

next = node.next;

node.next = cur.next;

cur.next = node;

node = next;

}

cur = tail;

}

cur.next = node;

return dummy.next;

}

}