用javascript写一个链表反转的代码，将链表后半段进行反转。如果链表长2n，反转n+1到2n部分（n为正整数）。如果链表长2n+1，反转n+1到2n+1的部分。

function reverseLinkedList(head) {
  if (!head || !head.next) return head;
  let prev = null;
  let curr = head;
  while (curr) {
    let next = curr.next;
    curr.next = prev;
    prev = curr;
    curr = next;
  }
  return prev;
}

这是反转整个链表的代码。要反转后半段，可以先使用快慢指针找到链表的中点，再将后半段反转，最后将反转后的后半段连接到前半段的末尾。

例如：

function reverseLinkedListInHalf(head) {
  if (!head || !head.next) return head;

  // Find the middle of the linked list
  let slow = head;
  let fast = head;
  while (fast && fast.next) {
    slow = slow.next;
    fast = fast.next.next;
  }

  // Reverse the second half of the linked list
  let prev = null;
  let curr = slow;
  while (curr) {
    let next = curr.next;
    curr.next = prev;
    prev = curr;
    curr = next;
  }

  // Connect the first half and the reversed second half
  head.next = prev;
  return head;
}