Merge pull request #102 from TheAlgorithms/add-sum-digits-different-approach

Add Digits: mathematical - digital root approach

DIRECTORY.md

@@ -40,6 +40,7 @@
 ## Maths
   * [Abs](https://github.com/TheAlgorithms/Ruby/blob/master/maths/abs.rb)
   * [Abs Test](https://github.com/TheAlgorithms/Ruby/blob/master/maths/abs_test.rb)
+  * [Add Digits](https://github.com/TheAlgorithms/Ruby/blob/master/maths/add_digits.rb)
   * [Aliquot Sum](https://github.com/TheAlgorithms/Ruby/blob/master/maths/aliquot_sum.rb)
   * [Aliquot Sum Test](https://github.com/TheAlgorithms/Ruby/blob/master/maths/aliquot_sum_test.rb)
   * [Binary To Decimal](https://github.com/TheAlgorithms/Ruby/blob/master/maths/binary_to_decimal.rb)

data_structures/arrays/add_digits.rb

@@ -33,10 +33,10 @@ def add_digits(num)
   add_digits(sum)
 end
 
-# puts(add_digits(38))
+puts(add_digits(38))
 # # => 2
 
-# puts(add_digits(284))
+puts(add_digits(284))
 # # => 5
 
 #
@@ -58,4 +58,4 @@ puts(add_digits(38))
 # => 2
 
 puts(add_digits(284))
-# => 5
+# => 5

maths/add_digits.rb

@@ -0,0 +1,35 @@
+# 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: 3 + 8 = 11, 1 + 1 = 2.
+
+#
+# Approach: Mathematical: Digital Root
+#
+# https://en.wikipedia.org/wiki/Digital_root
+# https://brilliant.org/wiki/digital-root/
+#
+
+#
+# Complexity Analysis
+#
+# Time Complexity: O(1).
+# Space Complexity: O(1).
+
+def add_digits(num)
+  return 0 if num == 0
+  return 9 if num % 9 == 0
+
+  num % 9
+end
+
+puts(add_digits(38))
+# # => 2
+
+puts(add_digits(284))
+# # => 5