Completed fibonacci challenge

lib/fibonacci.rb

@@ -4,6 +4,60 @@
 # e.g. 1st fibonacci number is 1
 # ....
 # e.g. 6th fibonacci number is 8
+# def fibonacci(n)
+#   if n.nil?
+#     raise ArgumentError, "Cannot be nil"
+#   end
+#
+#   if n.to_s.include?("-")
+#     raise ArgumentError, "Cannot be negative"
+#   end
+#   # From 0 to N
+#   # Each number is the sum of the two numbers before it
+#   # Generate array of numbers
+#   # Look at prior to numbers, add them together
+#   # 0 and 1 do not have prior two entries to add together
+#   numbers = [*0..n]
+#   # Start at index 2, manually add in 0 and 1
+#   result = [0, 1]
+#   i = 2
+#   while i < numbers.length
+#     # First number
+#     a = result[i - 1]
+#     # Second number
+#     b = result[i - 2]
+#     # Push sum of a and b to result array
+#     result << (a + b)
+#     # Increase iterator
+#     i += 1
+#   end
+#   # Return result at n index
+#   return result[n]
+# end
+
+# Linear runtime , O(1) space complexity
+
+# Recursive attempt
+
 def fibonacci(n)
-  raise NotImplementedError
+  if n.nil?
+      raise ArgumentError, "Cannot be nil"
+    end
+
+  if n.to_s.include?("-")
+    raise ArgumentError, "Cannot be negative"
+  end
+
+  # When n is less than 0 or 1, number is returned
+  if n < 2
+    return n
+    # 0 returns 0, 1 returns 1
+  end
+  # Otherwise, returns sum of prior two numbers
+  # Enters recursion
+  return fibonacci( n-1 ) + fibonacci( n-2)
+
 end
+
+# Time Complexity
+# Recursive - Takes much longer :(