add a solution of a issue#2

Author: Neodelf

2/add_two_numbers.rb

@@ -0,0 +1,30 @@
+# 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)
+    ll_common = ListNode.new(nil)
+    ll = ll_common
+    next_is_present = true
+    while next_is_present
+        next_is_present = l1&.next || l2&.next
+        ll_val = l1&.val.to_i + l2&.val.to_i + ll&.val.to_i
+        int = ll_val/10
+
+        ll.val = int > 0 ? ll_val - 10 : ll_val
+        ll.next = ListNode.new(int) if next_is_present || int > 0
+
+        l1 = l1.next if l1
+        l2 = l2.next if l2
+        ll = ll.next
+    end
+    ll_common
+end