Adding solution for 268

Ruby/268.rb

@@ -0,0 +1,32 @@
+# 268. Missing Number
+
+# @param {Integer[]} nums
+# @return {Integer}
+def missing_number(nums)
+  ns = nums.size()
+
+  nums = nums.sort()
+
+  i = 0
+  while i < ns
+    return i if nums[i] != i
+    i += 1
+  end
+  return ns
+end
+
+def main()
+  def r(n, exp)
+    puts "missing_number(#{n}) = #{missing_number(n)} | exp: #{exp}"
+  end
+
+  r([3,0,1], 2)
+  r([0,1], 2)
+  r([9,6,4,2,3,5,7,0,1], 8)
+
+end
+
+
+if __FILE__ == $0
+  main()
+end