diff --git a/lib/factorial.rb b/lib/factorial.rb
index 6a4a408..355c45a 100644
--- a/lib/factorial.rb
+++ b/lib/factorial.rb
@@ -1,6 +1,15 @@
 # Computes factorial of the input number and returns it
-# Time complexity: ?
-# Space complexity: ?
+# Time complexity: O(n)
+# Space complexity: O(1)
 def factorial(number)
-  raise NotImplementedError
+  if number == nil
+    raise ArgumentError
+  end
+
+  i = 1
+  while number > 0
+    i = number * i
+    number -= 1
+  end
+  return i
 end