Build a key-value data structure which can perform lookup and rangeLookup(key1, key2)

Build a key-value data structure which can perform following 2 functions 
- lookup 

- rangeLookup(key1, key2)

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We can implement it by using a combination of HashMap and BST. HashMap will contain entry for key-value pair and BST only keys. All keys will be maintained in BST (sorted order). 

lookup(Key): Make a simple lookup in to HashMap 
rangeLookup() - It will be in two parts - 
Keys[] rage(Key1, Key 2) from BST 
Entry[] lookups(Keys[]) in HashMap