odd_even_linked_list.dart

class Solution {
  ListNode? oddEvenList(ListNode? head) {
    if (head == null) {
      return null;
    }
    var odd = head;
    var even = head.next;
    var evenHead = even;
    while (even != null && even.next != null) {
      odd.next = even.next;
      odd = odd.next!;
      even.next = odd.next;
      even = even.next;
    }
    odd.next = evenHead;
    return head;
  }
}

class ListNode {
  int val;
  ListNode? next;

  ListNode([this.val = 0, this.next]);
}