// Definition for singly-linked list.
#[derive(PartialEq, Eq, Clone, Debug)]
pub struct ListNode {
  pub val: i32,
  pub next: Option<Box<ListNode>>
}

impl ListNode {
  #[inline]
  fn new(val: i32) -> Self {
    ListNode {
      next: None,
      val
    }
  }
}

pub fn make_list(arr: Vec<i32>) -> Option<Box<ListNode>> {
    let mut head: Option<Box<ListNode>> = None;
    let mut current = &mut head;

    for &value in arr.iter() {
        let new_node = Box::new(ListNode::new(value));
        if current.is_none() {
            *current = Some(new_node);
        } else {
            current.as_mut().unwrap().next = Some(new_node);
            current = &mut current.as_mut().unwrap().next;
        }
    }

    head
}