Add Two Numbers

算是数据结构题？按位加就行，Rust 可以写出比较优美的状态机 On & O1

// 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 {
    pub fn add_two_numbers(l1: Option<Box<ListNode>>, l2: Option<Box<ListNode>>) -> Option<Box<ListNode>> {
        let mut res = None;
        let mut current = &mut res;

        let mut t = (&l1, &l2, 0, 0);
        loop {
            t = match t {
                (Some(p), Some(q), res, carry) => 
                    (&p.next, &q.next, (p.val + q.val + carry)% 10, (p.val + q.val + carry)/ 10),
                (None, Some(q), res, carry) =>
                    (&None, &q.next, (q.val + carry)% 10, (q.val + carry)/ 10),
                (Some(p), None, res, carry) =>
                    (&p.next, &None, (p.val + carry)% 10, (p.val + carry)/ 10),
                (None, None, res, carry) if carry == 1 =>
                    (&None, &None, 1, 0),
                (None, None, res, _) => break
            };
            *current = Some(Box::new(ListNode::new(t.2)));
            current = &mut current.as_mut().unwrap().next;
        }

        res
    }
}