diff --git a/lib/factorial.rb b/lib/factorial.rb
index 6a4a408..2f41249 100644
--- a/lib/factorial.rb
+++ b/lib/factorial.rb
@@ -1,6 +1,12 @@
 # Computes factorial of the input number and returns it
-# Time complexity: ?
-# Space complexity: ?
+# Time complexity: O(n), where n represents the number. The while loop runs n - 1 times, so the time complexity does depend on n.
+# Space complexity: Constant. Regardless of how big the number is, there are only two variables inside the method.
 def factorial(number)
-  raise NotImplementedError
+  raise ArgumentError unless number
+  factorial = 1
+  while number > 1
+    factorial *= number
+    number -= 1
+  end
+  return factorial
 end