1
var reorderList = function (head) {
2
  const dummyL = new ListNode(-1);
3
  const dummyR = new ListNode(-1);
4

5
  let currL = dummyL;
6
  let currR = dummyR;
7
  let past = false;
8

9
  let fast = head;
10
  let slow = head;
11
  while (slow) {
12
    if (!fast?.next) {
13
      past = true;
14
    }
15

16
    if (past) {
17
      currR.next = slow;
18
      currR = slow;
19
    } else {
20
      currL.next = slow;
21
      currL = slow;
22
    }
23

24
    if (fast) {
25
      fast = fast.next?.next || null;
26
    }
27
    slow = slow.next;
28
  }
29
  currL.next = null;
30
  currR.next = null;
31

32
  dummyR.next = reverse(dummyR.next);
33
  return merge(dummyL.next, dummyR.next);
34
};
35

36
const merge = (l, r) => {
37
  const dummy = new ListNode(-1);
38

39
  let currL = l;
40
  let currR = r;
41
  let last = dummy;
42

43
  let count = 0;
44
  while (currL && currR) {
45
    if (count % 2 === 0) {
46
      last.next = currL;
47
      last = currL;
48
      currL = currL.next;
49
    } else {
50
      last.next = currR;
51
      last = currR;
52
      currR = currR.next;
53
    }
54

55
    count++;
56
  }
57

58
  last.next = currL || currR;
59
  return dummy.next;
60
};
61

62
const reverse = (head) => {
63
  let prev = null;
64
  let curr = head;
65
  while (curr) {
66
    const tempPrev = curr.next;
67
    curr.next = prev;
68
    prev = curr;
69
    curr = tempPrev;
70
  }
71

72
  return prev;
73
};