Hashtable with both values as key

Question

Is there a hashing based data structure where I can search an item in O(1) time on both key and value.

This can be achieved by adding duplicate entry in the list for each key value par by reversing key and value, but it will take double the space.

This kind of data structure might be useful in some scenarios: like I want to store opening and closing parenthesis in a map and while parsing the string, I can just check in the map if the key is present without worrying about whether it is opening-closing map or closing-opening map or without storing duplicate.

I hope I am clear enough!!

Answer 1

Data structure that fulfills your needs is called bidirectional map.

I suppose that you are looking for the existing implementation, not for the pointers how to implement it :) Since you didn't specify the programming language, this is the current situation for Java - there is no such data structure in Java API. However, there is Google Guava's bi-directional map interface with several implementations. From the docs:

A bimap (or "bidirectional map") is a map that preserves the uniqueness of its values as well as that of its keys. This constraint enables bimaps to support an "inverse view", which is another bimap containing the same entries as this bimap but with reversed keys and values.

Alternatively, there is BidiMap from Apache Collections.

For C++, have a look at Boost.Bimap .

For Python, have a look at bidict .

In C#, as well as in other languages, there does not exist an official implementation, but that's where Jon Skeet comes in .

Answer 2

You're searching for a bidirectional map . Here is an article describing the implementation in c++. Note though that a bidirectional map is basically two maps merged into a single object. There isn't any more efficient solution than this though, for a simple reason:

a map is basically an unconnected directed graph of (key,value)-pairs. Each pair is represented by an edge. If you want the map to be bidirectional you'll wind up with twice as many edges, thus doubling the amount of required memory.

C++ and Java STL don't provide any classes for this purpose though. In Java you can use Googles Guava library, in C++ the boost-library provides bi-directional maps.