How to create a simple prefix index in Java?

Question

I have big set of urls and I want to implement an autocompletion. I don\'t like the complexity of the naive approach as it is linear with the set size:

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Answer 1

If you need to efficiently find prefixes of strings, use a Trie, a data structure designed precisely for that purpose:

A trie, or prefix tree, is an ordered tree data structure that is used to store an associative array where the keys are usually strings. Unlike a binary search tree, no node in the tree stores the key associated with that node; instead, its position in the tree defines the key with which it is associated. All the descendants of a node have a common prefix of the string associated with that node, and the root is associated with the empty string

Two links with sample implementations.