Added list operations

ziffers/common.py

@@ -1,2 +1,14 @@
-def flatten(arr) -> list: 
-    return flatten(arr[0]) + (flatten(arr[1:]) if len(arr) > 1 else []) if type(arr) is list else [arr]
+def flatten(arr) -> list:
+    ''' Flattens array'''
+    return flatten(arr[0]) + (flatten(arr[1:]) if len(arr) > 1 else []) if type(arr) is list else [arr]
+
+def sum_dict(arr) -> dict:
+    ''' Sums array of dicts: [{a:3,b:3},{b:1}] -> {a:3,b:4} '''
+    result = arr[0]
+    for hash in arr[1:]:
+         for key in hash.keys():
+            if key in result:
+                result[key] = result[key] + hash[key]
+            else:
+                result[key] = hash[key]
+    return result