Added memoization to the existing code.

Author: eyuell21

Sprint-2/improve_with_caches/fibonacci/fibonacci.py

@@ -1,4 +1,8 @@
-def fibonacci(n):
+def fibonacci(n, memo={}):
+    if n in memo:
+        return memo[n]
     if n <= 1:
         return n
-    return fibonacci(n - 1) + fibonacci(n - 2)
+    
+    memo[n] = fibonacci(n - 1, memo) + fibonacci(n - 2, memo)
+    return memo[n]