migrate 392 questions to pattern format

backend/data/questions/design-hashset.yaml

@@ -7,7 +7,8 @@ categories:
   - arrays
   - hash-tables
 patterns:
-  - heap
+  - slug: hashing
+    is_optimal: true
 
 function_signature: "class MyHashSet:\n    def __init__(self): ...\n    def add(self, key: int) -> None: ...\n    def remove(self, key: int) -> None: ...\n    def contains(self, key: int) -> bool: ..."