Refactor fibonacci function to use a global memoization dictionary

Author: eyuell21

Sprint-2/improve_with_caches/fibonacci/fibonacci.py

@@ -1,8 +1,10 @@
-def fibonacci(n, memo={}):
-    if n in memo:
-        return memo[n]
+_memo = {}
+
+def fibonacci(n):
+    if n in _memo:
+        return _memo[n]
     if n <= 1:
         return n
 
-    memo[n] = fibonacci(n - 1, memo) + fibonacci(n - 2, memo)
-    return memo[n]
+    _memo[n] = fibonacci(n - 1) + fibonacci(n - 2)
+    return _memo[n]