This site runs best with JavaScript enabled.

Prefix Tree

Robin Kim

August 14, 2014

By reading this post, you will be able to answer the following questions:

  • What is a prefix tree? (Tree that groups together elements with common beginnings.)
  • How do I navigate through one? (One piece of the data at a time!)
  • What can I use a prefix tree for? (Autocompletion: search engine, T9 texting, spellcheck, etc.)
  • Can I see one in action? (Yes!)

Before continuing, you may want to be comfortable with the following topics:

  • What is a tree (data structure)?
  • What are its basic characteristics?
  • How does navigation through a tree and its child nodes work?

Slide 1: Introduction

I will be using a 5-minute presentation I created for my Hack Reactor cohort to act as a visual aid for this post. You can view the slideshow here, and a demo of my implementation of a prefix tree is at the end of the post.

Slide 2: Characteristics

A prefix tree is a data structure that is often used to store words. It groups elements with similar beginnings together so they share ancestor nodes. A prefix tree has quick lookup times for words, so it's a great choice for storing dictionary words.

A suffix tree groups elements with similar endings together.

Slide 3: Example A

If you want to find the word tap, then:

  1. Start at the root node, which doesn't hold any data itself. (It has references to other child nodes instead.)
  2. Ask the root node: "Hey, do you know where I can find t?"
  3. The root node replies: "Yes! It's right here!"
  4. Once you navigate to the t node, ask that node: "Where can I find a?"
  5. Once we reach a, ask for p.
  6. After arriving at the p node, we can ask it: "Are we at the end of a word (EOW)?"
  7. We get a response: "Yes!"
  8. Congratulations, we've successfully found tap in our prefix tree.

Slide 4: Example B

Here, we look for tea:

  1. Start at the root node.
  2. Navigate to t and ask: "Where can I find e?"
  3. Arrive at te! (This prefix tree is drawn so that it keeps track of all of the preceding letters.)
  4. Ask for a, and arrive at tea.
  5. The final tea node informs us that it is the end of a word.
  6. Success!

Slide 5: Possible Uses

A prefix tree can be used to store autocompletion results for:

  • Search engines (which also rank the relevance of the autocompletion suggestions)
  • T9 texting
  • Spellcheck

Slide 6: Example (Autocompletion)

In this example, we will use our given prefix tree to look for autocompletion suggestions for the string t. Put in another way, what words start with the letter t?

  1. We start at the root node and ask: "Where can I find the letter t?"
  2. The root node points us to the t node, at which point we must begin our search for autocompletion suggestions.
  3. We can do this by (recursively) navigating to ALL child nodes of t and creating a list of those which "end of word" attribute is set to true.
  4. to, tea, ted, and ten are valid words, which are added to our results, while te is not, hence ommitted.

Slide 7: My Implementation

The listed property and methods summarize my implementation of prefix tree. I've preloaded my prefix tree with almost 5,000 common English words.

Slide 8: Time for a Demo!

Go to for a working demo.

Looking for my code? It's right here!

Share article