Trie interview questions

A trie, also called a prefix tree, is a tree structured for storing words character by character. Instead of storing entire words in individual nodes, each step down a branch represents a single letter. Words that share the same beginning, like car, card, and care, share the exact same starting path down the tree. A special boolean marker sits at the end of each valid word to show that a complete word terminates at that letter.

When to reach for it

Reach for a trie when questions involve prefix lookups, dictionary word searches, autocomplete engines, or matching prefixes against a body of text. Prompts asking whether any word in a dictionary begins with a given prefix, or searching for words on a Boggle board grid, point directly to a trie. It also applies to bitwise tasks, such as finding the maximum XOR pair among integers by treating numbers as binary prefixes.

How to think about it

Represent each trie node with an array or hash map of child links, plus a boolean flag marking if a complete word ends at that node. When inserting, start at the root and walk down character by character, creating new child nodes whenever a path does not exist yet, then mark the final node as a word ending. When searching, follow the characters; if any child link is missing, the word or prefix does not exist. If all characters match, check the boolean flag to distinguish between a full word and a partial prefix.

What each operation costs