Add cache to fibonacci

Author: katarzynakaz

Sprint-2/improve_with_caches/fibonacci/fibonacci.py

@@ -1,4 +1,12 @@
-def fibonacci(n):
+def fibonacci(n, cache=None):
+    if cache is None:
+        cache = {}
+
+    if n in cache:
+        return cache[n]
+
     if n <= 1:
         return n
-    return fibonacci(n - 1) + fibonacci(n - 2)
+
+    cache[n] = fibonacci(n - 1, cache) + fibonacci(n - 2, cache)
+    return cache[n]