K个一组反转链表

var reverseGroup = (head, k) => {
    if(head === null) {
      return null
  }
  let a = head;
  let b = head;
  for(let i = 0;i < k;i++) {
      if(b === null) {
        return head
    }
    b = b.next
  }
  let newHead = reverse(a, b)
  a.next = reverseGroup(b, k)
  return newHead
}
//反转[a, b)
function reverse(a, b) {
    let pre = null;
  let cur = a;
  let nxt = a;
  while(cur !== b) {
      next = cur.next;
    cur.next = pre;
    pre = cur;
    cur = nxt;
  }
}