LRU Cache for Query Results

Maintain a cache with positive integer capacity. Process each operation atomically in the supplied completed serialization order:

• [1, key] performs get(key). Return the stored value, or -1 when the key is absent. A successful get makes the key most recently used.
• [2, key, value] performs put(key, value). Insert or update the key and make it most recently used. Updating an existing key does not change the cache size. If an insertion exceeds capacity, evict exactly the least recently used key.

Return one string per operation: the decimal result of each get and the literal string "null" for each put. Implement both operations in O(1) expected time.