Merge pull request TheAlgorithms#160 from jsca-kwok/jk-max-69-number

Add solution for max 69 number algorithm

Author: vbrazo

DIRECTORY.md

@@ -27,6 +27,7 @@
     * [Get Products Of All Other Elements](https://github.com/TheAlgorithms/Ruby/blob/master/data_structures/arrays/get_products_of_all_other_elements.rb)
     * [Good Pairs](https://github.com/TheAlgorithms/Ruby/blob/master/data_structures/arrays/good_pairs.rb)
     * [Intersection](https://github.com/TheAlgorithms/Ruby/blob/master/data_structures/arrays/intersection.rb)
+    * [Max 69 Number](https://github.com/TheAlgorithms/Ruby/blob/master/data_structures/arrays/max_69_number.rb)
     * [Next Greater Element](https://github.com/TheAlgorithms/Ruby/blob/master/data_structures/arrays/next_greater_element.rb)
     * [Remove Elements](https://github.com/TheAlgorithms/Ruby/blob/master/data_structures/arrays/remove_elements.rb)
     * [Richest Customer Wealth](https://github.com/TheAlgorithms/Ruby/blob/master/data_structures/arrays/richest_customer_wealth.rb)

data_structures/arrays/max_69_number.rb

@@ -0,0 +1,40 @@
+# Challenge name: Maximum 69 number
+#
+# Given a positive integer num consisting only of digits 6 and 9. 
+# Return the maximum number you can get by changing at most one digit (6 becomes 9, and 9 becomes 6).
+#
+# Example 1:
+# Input: num = 9669
+# Output: 9969
+# Explanation:
+# Changing the first digit results in 6669.
+# Changing the second digit results in 9969.
+# Changing the third digit results in 9699.
+# Changing the fourth digit results in 9666.
+# The maximum number is 9969.
+#
+# Example 2:
+# Input: num = 9996
+# Output: 9999
+# Explanation: Changing the last digit 6 to 9 results in the maximum number.
+
+#
+# Approach 1: Logical Approach
+# Explanation: Changing the first available 6 to a 9 will give the max number
+#
+def max_number(num)
+  arr = num.to_s.split('')
+
+  arr.each_with_index do |num, i|
+    if num == '6'
+      arr[i] = '9'
+      return arr.join.to_i
+    end
+  end
+end
+
+puts max_number(9669)
+# => 9969
+
+puts max_number(9996)
+# => 9999