0002.两数相加

https://leetcode.cn/problems/add-two-numbers/

方法一：模拟加法

时间复杂度 $O(n)$，空间复杂度 $O(1)$。

impl Solution {
    pub fn add_two_numbers(l1: Option<Box<ListNode>>, l2: Option<Box<ListNode>>) -> Option<Box<ListNode>> {
        let mut ans = ListNode::new(-1);
        let (mut l1, mut l2) = (l1, l2);
        let mut remainder = 0;
        let mut p = &mut ans;
        while l1.is_some() || l2.is_some() || remainder != 0 {
            if let Some(a) = l1 {
                l1 = a.next;
                remainder += a.val;
            }
            if let Some(b) = l2 {
                l2 = b.next;
                remainder += b.val;
            }
            p.next = Some(Box::new(ListNode::new(remainder % 10)));
            p = p.next.as_mut().unwrap().as_mut();
            remainder /= 10;
        }
        ans.next
    }
}