快速按值查找键，而无需创建反向哈希图？

Question

If I have a HashMap<KEY, VALUE> and I need fast look up of the key by the value is there any other approach besides creating a second HashMap<VALUE, KEY> that store the same data but using the value as the key?
Is there any approach/tick about this? If it makes a difference my interest is about String both as key and value
Note: I am on Java 7

Update:
I am not sure why the other question is a duplicate as I am asking a specific way on implementing this.
Unless the only/best way is a 2 way map I can't see why this is a duplicate

Answer 1

Short answer: no, there isn't.

You need two maps. If you want to use O(1) time for both, that means two hashmaps.

If you're worried about space, don't worry so much: you're just storing duplicate pointers, and not two strings.

Ie, you're just storing

HashMap<String* k, String* v> normal;
HashMap<String* k, String* v> inverse;

rather than entire strings. (Although pointers kind of don't exist in Java.)