From 7b57013dbb71855ed0b157a7758ce72214db831c Mon Sep 17 00:00:00 2001
From: Shamira <marshallscm@gmail.com>
Date: Sun, 21 Apr 2019 12:14:21 -0700
Subject: [PATCH] Fibonacci with Big O explanations.

---
 lib/fibonacci.rb | 17 ++++++++++++++---
 1 file changed, 14 insertions(+), 3 deletions(-)

diff --git a/lib/fibonacci.rb b/lib/fibonacci.rb
index 606755b..e17139d 100644
--- a/lib/fibonacci.rb
+++ b/lib/fibonacci.rb
@@ -5,8 +5,19 @@
 # ....
 # e.g. 6th fibonacci number is 8
 
-# Time complexity: ?
-# Space complexity: ?
+# Time complexity: O(n) with n being the input.
+# Space complexity: O(1)- constant because it take the same amount of space regardless of the input.
 def fibonacci(n)
-  raise NotImplementedError
+  if n == nil || n < 0
+    raise ArgumentError, "n cannot be nill or negaitve"
+  end
+
+  if n == 1 || n == 2
+    result = 1
+  elsif n == 0
+    result = 0
+  else
+    result = fibonacci(n - 1) + fibonacci(n - 2)
+  end
+  return result
 end