LRUCacheRetrieveObject.swift

class LRUCache<T: Hashable, U> {
  // ...
  
  /// Returns the element at the specified key. Nil if it doesn't exist.
    func retrieveObject(at key: T) -> U? {
        guard let existingNode = dictionary[key] else {
            return nil
        }

        linkedList.moveToHead(node: existingNode)
        return existingNode.payload.value
    }
}