Add single number challenge

data_structures/arrays/single_number.rb

@@ -0,0 +1,22 @@
+# Challenge name: Single Number
+#
+# Given a non-empty array of integers nums, every element appears twice
+# except for one. Find that single one.
+#
+# Follow up: Could you implement a solution with a linear runtime
+# complexity and without using extra memory?
+#
+# @param {Integer[]} nums
+# @return {Integer}
+
+def single_number(nums)
+end
+nums = [2, 2, 1]
+puts(single_number(nums))
+# Output: 1
+nums = [4, 1, 2, 1, 2]
+puts(single_number(nums))
+# Output: 4
+nums = [1]
+puts(single_number(nums))
+# Output: 1