diff --git a/lib/factorial.rb b/lib/factorial.rb
index 6a4a408..26368e2 100644
--- a/lib/factorial.rb
+++ b/lib/factorial.rb
@@ -1,6 +1,13 @@
 # Computes factorial of the input number and returns it
-# Time complexity: ?
-# Space complexity: ?
+# Time complexity: O(n) where n is the value of the input number
+# Space complexity: O(1)
 def factorial(number)
-  raise NotImplementedError
+  unless number
+    raise ArgumentError, "Non-number entered."
+  end
+  if number == 1 || number == 0
+    return 1
+  else
+    number * factorial(number - 1)
+  end
 end