diff --git a/lib/factorial.rb b/lib/factorial.rb
index 6a4a408..d04b494 100644
--- a/lib/factorial.rb
+++ b/lib/factorial.rb
@@ -1,6 +1,16 @@
 # Computes factorial of the input number and returns it
-# Time complexity: ?
-# Space complexity: ?
+# Time complexity: O(n) where n is the number
+# Space complexity: O(1)
 def factorial(number)
-  raise NotImplementedError
+  return 1 if number == 0
+  return 1 if number == 1
+  raise ArgumentError if number == nil
+  fact = 1
+  index = 1
+  while index < number
+    fact *= (index + 1)
+    index += 1
+  end
+
+  return fact
 end
diff --git a/specs/factorial_spec.rb b/specs/factorial_spec.rb
index de28055..89067ab 100644
--- a/specs/factorial_spec.rb
+++ b/specs/factorial_spec.rb
@@ -1,6 +1,6 @@
-require 'minitest/autorun'
-require 'minitest/reporters'
-require_relative '../lib/factorial'
+require "minitest/autorun"
+require "minitest/reporters"
+require_relative "../lib/factorial"
 
 describe "factorial" do
   describe "basic tests" do
@@ -8,6 +8,10 @@
       factorial(5).must_equal 120
     end
 
+    it "factorial(3) = 6" do
+      factorial(3).must_equal 6
+    end
+
     it "factorial(7) = 5040" do
       factorial(7).must_equal 5040
     end
@@ -17,7 +21,7 @@
   describe "edge cases" do
     # if the parameter is an object, check for nil
     it "nil object is not an integer" do
-      proc {factorial(nil)}.must_raise ArgumentError
+      proc { factorial(nil) }.must_raise ArgumentError
     end
 
     it "factorial(0) = 1" do