diff --git a/lib/factorial.rb b/lib/factorial.rb
index 6a4a408..ca76bfb 100644
--- a/lib/factorial.rb
+++ b/lib/factorial.rb
@@ -1,6 +1,17 @@
 # Computes factorial of the input number and returns it
-# Time complexity: ?
-# Space complexity: ?
-def factorial(number)
-  raise NotImplementedError
+# Time complexity: O(n), linear, with n being the number input.
+# Space complexity: O(n), linear, with n being the number input for the factorial.
+
+def factorial(num)
+  if num == nil || num < 0
+    raise ArgumentError, "Must input positive number."
+  end
+
+  factorial = 1
+
+  num.times do
+    factorial *= num
+    num = num - 1
+  end
+  return factorial
 end