Tries for Prefix Lookups
Store and query word prefixes fast.
Tries for Prefix Lookups is a free Coding Interview Prep lesson on CoddyKit — lesson 4 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Coding Interview Prep learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.
Storing Words Smartly
A trie is a tree that stores words by sharing common prefixes. It makes prefix questions lightning fast. 🌳
Why Not Just a Set
A set answers full-word lookups, but tries also answer prefix queries like does any word start with pre.
Nodes and Edges
Each node is a position in some word, and each edge is labeled by a character on the path from the root.
Children as a Dict
In Python the easiest node is a dict mapping a character to its child node. Clean and flexible.
root = {}Inserting a Word
To insert, walk character by character and create a child whenever one is missing.
node = root
for c in word:
node = node.setdefault(c, {})Marking Word Ends
After inserting, set an end flag so you can tell a full word from a mere prefix.
node['#'] = TrueSearching a Full Word
To search, follow the characters; if any step is missing, the word is absent. Then check the end flag.
for c in word:
if c not in node:
return False
node = node[c]Checking a Prefix
A prefix query is the same walk, but you skip the end-flag check. Reaching the last node means yes.
Time Complexity
Insert and lookup cost O(L), the word length, no matter how many words you stored. Length is what counts.
Counting Words by Prefix
Store a count at each node to instantly answer how many stored words share a given prefix.
Where Tries Help
Tries power autocomplete, dictionary checks, and XOR-max problems on bits. A staple of contest string tasks.
Quick Check
Confirm what a trie lookup actually costs.
Recap: Tries Wrapped Up
You can now build a trie, insert and search in O(L), and answer fast prefix and count queries. 🌟
Frequently asked questions
Is the “Tries for Prefix Lookups” lesson free?
Yes — the full text of “Tries for Prefix Lookups” is free to read here on the web, and the Coding Interview Prep course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Coding Interview Prep course, upgrade to CoddyKit PRO.
What will I learn in “Tries for Prefix Lookups”?
Store and query word prefixes fast. You practise Coding Interview Prep with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start Coding Interview Prep?
No prior experience is required. Coding Interview Prep on CoddyKit is structured for beginners through advanced learners; this is — lesson 4 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “Tries for Prefix Lookups” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this Coding Interview Prep lesson?
Yes. Every Coding Interview Prep lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- KMP Prefix Function
- Polynomial String Hashing
- Z-Function for Pattern Search
- Tries for Prefix Lookups