143. Reorder List

1function reorderList(head: ListNode | null): void {
2  if (!head) return;
3  let slow = head, fast = head;
4  while (fast.next && fast.next.next) { slow = slow.next!; fast = fast.next.next; }
5  let prev: ListNode | null = null, curr = slow.next;
6  slow.next = null;
7  while (curr) { const nx = curr.next; curr.next = prev; prev = curr; curr = nx; }
8  let a: ListNode | null = head, b = prev;
9  while (b) {
10    const an: ListNode | null = a!.next, bn: ListNode | null = b.next;
11    a!.next = b; b.next = an; a = an; b = bn;
12  }
13}

Move slow by 1 and fast by 2.

Space: play/pause · ←/→: step