add_two_numbers.rb

# Definition for singly-linked list.
# class ListNode
#     attr_accessor :val, :next
#     def initialize(val)
#         @val = val
#         @next = nil
#     end
# end

# @param {ListNode} l1
# @param {ListNode} l2
# @return {ListNode}
def add_two_numbers(l1, l2)
  l1_array = []
  l2_array = []
  level1 = l1
  level2 = l2
  while level1
    l1_array << level1.val
    level1 = level1.next
  end
  
  while level2
    l2_array << level2.val
    level2 = level2.next
  end
  
  l1_num = l1_array.reverse.join.to_i
  l2_num = l2_array.reverse.join.to_i
  
  final_num = l1_num + l2_num
  
  final_array = final_num.to_s.split("").map {|n| n.to_i}
  
  root_node = ListNode.new(nil)
  active_node = root_node 
  final_array.reverse.each_with_index do |n,i|
      active_node.val = n
      if i+1 < final_array.length
        active_node.next = ListNode.new(nil) 
        active_node = active_node.next
      end
  end
  
  return root_node
end