Plus one linked list

impl Solution {
  pub fn plus_one(head: Option<Box<ListNode>>) -> Option<Box<ListNode>> {
    fn add_one(node: &mut Option<Box<ListNode>>) -> i32 {
      match node {
        None => 1,
        Some(n) => {
          let carry = add_one(&mut n.next);
          let sum = n.val + carry;
          n.val = sum % 10;
          sum / 10
        }
      }
    }
    let mut head = head;
    let carry = add_one(&mut head);
    if carry > 0 {
      let mut new_head = Box::new(ListNode::new(carry));
      new_head.next = head;
      Some(new_head)
    } else {
      head
    }
  }
}