Add solution using recursion

DIRECTORY.md

@@ -7,6 +7,7 @@
     * [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)
+    * [Power of Two](https://github.com/TheAlgorithms/Ruby/blob/master/data_structures/arrays/power_of_two.rb)
     * [Remove Elements](https://github.com/TheAlgorithms/Ruby/blob/master/data_structures/arrays/remove_elements.rb)
     * [Sort Squares Of An Array](https://github.com/TheAlgorithms/Ruby/blob/master/data_structures/arrays/sort_squares_of_an_array.rb)
     * [Two Sum](https://github.com/TheAlgorithms/Ruby/blob/master/data_structures/arrays/two_sum.rb)

data_structures/arrays/power_of_two.rb

@@ -0,0 +1,61 @@
+# Arrays - Power of 2
+#
+# Given an integer n, return true if it is a power of two. Otherwise, return false.
+#
+# An integer n is a power of two, if there exists an integer x such that n == 2^x.
+#
+# Example 1:
+# Input: n = 1
+# Output: true
+# Explanation: 2^0 = 1
+#
+# Example 2:
+# Input: n = 16
+# Output: true
+# Explanation: 2^4 = 16
+#
+# Example 3:
+# Input: n = 3
+# Output: false
+#
+# Example 4:
+# Input: n = 4
+# Output: true
+#
+# Example 5:
+# Input: n = 5
+# Output: false
+#
+# Constraints: -231 <= n <= 231 - 1
+# @param {Integer} n
+# @return {Boolean}
+#
+# Approach 1: Recursion
+#
+# Time Complexity: O(1)
+#
+def is_power_of_two(n)
+  if n == 1
+    return true
+  elsif n%2 == 0
+    is_power_of_two(n/2)
+  else
+    return false
+  end
+end
+
+n = 1
+# Output: true
+puts is_power_of_two(n)
+n = 16
+# Output: true
+puts is_power_of_two(n)
+n = 3
+# Output: false
+puts is_power_of_two(n)
+n = 4
+# Output: true
+puts is_power_of_two(n)
+n = 5
+# Output: false
+puts is_power_of_two(n)