给你单链表的头指针 head 和两个整数 left 和 right ，其中 left <= right 。请你反转从位置 left 到位置 right 的链表节点，返回 反转后的链表 。

思路

  1. 添加 dummyHead
  2. 找到 left 节点的前驱节点 keyPrev
  3. keyTail = leftNode，keyTail 为翻转后的链表段尾节点
  4. 翻转过程中始终保证链表的连续性，keyTail.next = remainLink
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var reverseBetween = function (head, left, right) {
  const dummyHead = new ListNode()
  dummyHead.next = head
  let keyPrev = dummyHead
  for (let i = 1; i < left; ++i) {
    keyPrev = keyPrev.next
  }
  let cur = keyPrev.next
  let keyTail = cur
  let newLink = null
  let steps = right - left + 1
  while (cur && steps--) {
    const remainLink = cur.next
    cur.next = newLink
    newLink = cur
    keyTail.next = remainLink
    cur = remainLink
  }
  keyPrev.next = newLink
  return dummyHead.next
}