Add Two Numbers
<p>You are given two <strong>non-empty</strong> linked lists representing two non-negative integers. The digits are stored in <strong>reverse order</strong>, and each of their nodes contains a single digit. Add the two numbers and return the sum&nbsp;as a linked list.</p>

<p>You may assume the two numbers do not contain any leading zero, except the number 0 itself.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://fastly.jsdelivr.net/gh/doocs/leetcode@main/solution/0000-0099/0002.Add%20Two%20Numbers/images/addtwonumber1.jpg" style="width: 483px; height: 342px;" />
<pre>
<strong>Input:</strong> l1 = [2,4,3], l2 = [5,6,4]
<strong>Output:</strong> [7,0,8]
<strong>Explanation:</strong> 342 + 465 = 807.
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> l1 = [0], l2 = [0]
<strong>Output:</strong> [0]
</pre>

<p><strong class="example">Example 3:</strong></p>

<pre>
<strong>Input:</strong> l1 = [9,9,9,9,9,9,9], l2 = [9,9,9,9]
<strong>Output:</strong> [8,9,9,9,0,0,0,1]
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li>The number of nodes in each linked list is in the range <code>[1, 100]</code>.</li>
	<li><code>0 &lt;= Node.val &lt;= 9</code></li>
	<li>It is guaranteed that the list represents a number that does not have leading zeros.</li>
</ul>
Recursion; Linked List; Math
/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     public var val: Int
 *     public var next: ListNode?
 *     public init() { self.val = 0; self.next = nil; }
 *     public init(_ val: Int) { self.val = val; self.next = nil; }
 *     public init(_ val: Int, _ next: ListNode?) { self.val = val; self.next = next; }
 * }
 */
class Solution {
    func addTwoNumbers(_ l1: ListNode?, _ l2: ListNode?) -> ListNode? {
        var dummy = ListNode.init()
        var carry = 0
        var l1 = l1
        var l2 = l2
        var cur = dummy
        while l1 != nil || l2 != nil || carry != 0 {
            let s = (l1?.val ?? 0) + (l2?.val ?? 0) + carry
            carry = s / 10
            cur.next = ListNode.init(s % 10)
            cur = cur.next!
            l1 = l1?.next
            l2 = l2?.next
        }
        return dummy.next
    }
}
