A data structure for a phone book such that it can search a number by name and also search a name by number

Question

Do you know a solution to the following interview question?

Design a data structure for a phone book that can safely and efficiently search a number

Answer 1

Such data structures are known as multi-index containers. They are not very common in most programming languages, because the interface can get quite complex. There are implementations in Java, C# and - most prominently - C++ with the Boost library, see Boost.MultiIndex Documentation, in particular example 4 about bidirectional maps:

A bidirectional map is a container of (const FromType,const ToType) pairs such that no two elements exists with the same first or second component (std::map only guarantees uniqueness of the first component). Fast lookup is provided for both keys. The program features a tiny Spanish-English dictionary with online query of words in both languages.

The basic idea of multi-index containers is that a lot of containers store their elements in nodes that contain pointers/references to other nodes (e.g. double linked lists). Instead of only storing the pointers/references for a single container, a node contains the links for several index structures. This works at least with linked lists, sorted trees and unique hash indices. And the implementation is very efficient, because only one memory allocation is required per element.