Ruby

1# Definition for singly-linked list.
2# class ListNode
3#     attr_accessor :val, :next
4#     def initialize(val = 0, _next = nil)
5#         @val = val
6#         @next = _next
7#     end
8# end
9# @param {ListNode} l1
10# @param {ListNode} l2
11# @return {ListNode}
12def add_two_numbers(l1, l2)
13    num1 = l1.val
14    num2 = l2.val
15    
16    pow = 1
17    while l1.next
18        l1 = l1.next
19        num1 = (l1.val) * (10**pow) + num1
20        pow += 1
21    end 
22    
23    pow = 1
24    while l2.next
25        l2 = l2.next
26        num2 = (l2.val) * (10**pow) + num2
27        pow += 1
28    end 
29        
30    num3 = num1+num2
31    l3 = ListNode.new(num3%10)
32    curr = l3
33    num3 /= 10
34    while num3 > 0
35        curr.next = ListNode.new(num3%10)
36        num3 /= 10
37        curr = curr.next
38    end 
39    
40    l3
41end