Add add digits challenge

DIRECTORY.md

@@ -4,6 +4,7 @@
 
 ## Data Structures
   * Arrays
+    * [Add Digits](https://github.com/TheAlgorithms/Ruby/blob/master/data_structures/arrays/add_digits.rb)
     * [Find All Duplicates In An Array](https://github.com/TheAlgorithms/Ruby/blob/master/data_structures/arrays/find_all_duplicates_in_an_array.rb)
     * [Find The Highest Altitude](https://github.com/TheAlgorithms/Ruby/blob/master/data_structures/arrays/find_the_highest_altitude.rb)
     * [Get Products Of All Other Elements](https://github.com/TheAlgorithms/Ruby/blob/master/data_structures/arrays/get_products_of_all_other_elements.rb)

data_structures/arrays/add_digits.rb

@@ -0,0 +1,24 @@
+# Challenge name: Add Digits
+#
+# Given a non-negative integer num, repeatedly add all its digits until the result has only one digit.
+#
+# Example:
+#
+# Input: 38
+# Output: 2
+# Explanation: The process is like: 3 + 8 = 11, 1 + 1 = 2.
+#              Since 2 has only one digit, return it.
+#
+# Follow up:
+# Could you do it without any loop/recursion in O(1) runtime?
+# @param {Integer} num
+# @return {Integer}
+
+#
+# Approach 1: Brute Force
+#
+def add_digits(num)
+end
+
+puts(add_digits(38))
+# Output: 2