ll-sum.rb

class ListNode
  attr_accessor :val, :next
  def initialize(val)
     @val  = val
     @next = nil
  end
end

@carry = false

def add_nodes(l1,l2)
  total = l1.val + l2.val

  if @carry
    total += 1
    @carry = false
  end

  if total > 9
    total  = total % 10
    @carry = true
  end

  create_node(total)
end

def add_two_numbers(l1, l2)
    @carry = false
    add_two_numbers_real(l1, l2)
end

def add_two_numbers_real(l1, l2)
  return create_node(1) if @carry and !l1
  return unless l1 or l2

  l2 = create_node(0) unless l2
  l1 = create_node(0) unless l1

  node = add_nodes(l1, l2)

  node.next = add_two_numbers_real(l1.next, l2.next)

  node
end

module List
  def create_list(values)
    recursive_append_multiple(values)
    # head = create_node(values.shift)
    # values.each { |n| head  = append_node(head, n) }
    # head
  end

  def create_node(value)
    ListNode.new value
  end

  def recursive_append_multiple(values, idx = 0)
    return if idx >= values.size

    node = create_node(values[idx])
    node.next = recursive_append_multiple(values, idx+1)
    node
  end

  # Loop until node.next = nil
  def append_node(node, value)
    node.tap {
      while node.next
        node = node.next
      end
      node.next = create_node(value)
    }
  end
end

include List

list_1 = create_list [3,2,1]
list_2 = create_list [3,2,1]

one  = create_list [1]
five = create_list [5]

list_3 = create_list [1,8]
zero   = create_list [0]

p add_two_numbers one, five

p add_two_numbers five, five

p add_two_numbers list_1, list_2

p add_two_numbers list_3, zero

p add_two_numbers create_list([9]), create_list([9])

p add_two_numbers create_list([9,9]), create_list([1])