Solution.rs

// 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
//     }
//   }
// }
impl Solution {
    fn reverse(mut head: Option<Box<ListNode>>) -> Option<Box<ListNode>> {
        let mut pre = None;
        while let Some(mut node) = head {
            let next = node.next.take();
            node.next = pre.take();
            pre = Some(node);
            head = next;
        }
        pre
    }

    pub fn add_two_numbers(
        mut l1: Option<Box<ListNode>>,
        mut l2: Option<Box<ListNode>>
    ) -> Option<Box<ListNode>> {
        l1 = Self::reverse(l1);
        l2 = Self::reverse(l2);
        let mut dummy = Some(Box::new(ListNode::new(0)));
        let mut cur = &mut dummy;
        let mut sum = 0;
        while l1.is_some() || l2.is_some() || sum != 0 {
            if let Some(node) = l1 {
                sum += node.val;
                l1 = node.next;
            }
            if let Some(node) = l2 {
                sum += node.val;
                l2 = node.next;
            }
            cur.as_mut().unwrap().next = Some(Box::new(ListNode::new(sum % 10)));
            cur = &mut cur.as_mut().unwrap().next;
            sum /= 10;
        }
        Self::reverse(dummy.unwrap().next.take())
    }
}