AddTwoNumbers.swift

/**
 * Question Link: https://leetcode.com/problems/add-two-numbers/
 * Primary idea: use carry and iterate through both linked lists
 * Time Complexity: O(n), Space Complexity: O(1)
 *
 * Definition for singly-linked list.
 * public class ListNode {
 *     public var val: Int
 *     public var next: ListNode?
 *     public init(_ val: Int) {
 *         self.val = val
 *         self.next = nil
 *     }
 * }
 */

class AddTwoNumbers {
    func addTwoNumbers(l1: ListNode?, _ l2: ListNode?) -> ListNode? {
        var carry = 0, l1 = l1, l2 = l2
        let dummy = ListNode(0)
        var node = dummy
        
        while l1 != nil || l2 != nil || carry != 0 {
            if l1 != nil {
                carry += l1!.val
                l1 = l1!.next
            }
            if l2 != nil {
                carry += l2!.val
                l2 = l2!.next
            }
            
            node.next = ListNode(carry % 10)
            node = node.next!
            
            carry = carry / 10
        }
        
        return dummy.next 
    }
}