# The Trie data structure

## #data-structures series

Posted on February 9, 2016

The #data-structures series is a collection of posts about reimplemented data structures in JavaScript.

If you are not familiar with data structures, a quick introduction and the full list of reimplemented data structures can be found in the introduction post of the series on data structures in JavaScript.

If you feel comfortable with the concept of each data structure and only want to see the code, have a look at the summary post of the series. It removes all explanations and contains only the JavaScript code for all data structures discussed in the series.

## Get the code on Github

Of course, all the code can also be found on Github in the repository data-structures-in-javascript.

# The Trie data structure

## Definition

A trie, also called digital tree and sometimes radix tree or prefix tree (as they can be searched by prefixes), is an ordered tree data structure that is used to store a dynamic set or 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. Values are not necessarily associated with every node. Rather, values tend only to be associated with leaves, and with some inner nodes that correspond to keys of interest. From Wikipedia

## Complexity

Average
Access Search Insertion Deletion
O(k) O(k) O(k) O(k)

where k is the word length.

To get a full overview of the time and space complexity of the Trie data structure, have a look to this excellent Big O cheat sheet.

## The code

``````function Node(data) {
this.data = data;
this.isWord = false;
this.prefixes = 0;
this.children = {};
}

function Trie() {
this.root = new Node('');
}

if(!this.root) {
return null;
}
};
if(!node || !word) {
return null;
}
node.prefixes++;
var letter = word.charAt(0);
var child = node.children[letter];
if(!child) {
child = new Node(letter);
node.children[letter] = child;
}
var remainder = word.substring(1);
if(!remainder) {
child.isWord = true;
}
};
Trie.prototype.remove = function(word) {
if(!this.root) {
return;
}
if(this.contains(word)) {
this._removeNode(this.root, word);
}
};
Trie.prototype._removeNode = function(node, word) {
if(!node || !word) {
return;
}
node.prefixes--;
var letter = word.charAt(0);

var child = node.children[letter];
if(child) {
var remainder = word.substring(1);
if(remainder) {
if(child.prefixes === 1) {
delete node.children[letter];
} else {
this._removeNode(child, remainder);
}
} else {
if(child.prefixes === 0) {
delete node.children[letter];
} else {
child.isWord = false;
}
}
}
};
Trie.prototype.contains = function(word) {
if(!this.root) {
return false;
}
return this._contains(this.root, word);
};
Trie.prototype._contains = function(node, word) {
if(!node || !word) {
return false;
}
var letter = word.charAt(0);
var child = node.children[letter];
if(child) {
var remainder = word.substring(1);
if(!remainder && child.isWord) {
return true;
} else {
return this._contains(child, remainder);
}
} else {
return false;
}
};
Trie.prototype.countWords = function() {
if(!this.root) {
return console.log('No root node found');
}
var queue = [this.root];
var counter = 0;
while(queue.length) {
var node = queue.shift();
if(node.isWord) {
counter++;
}
for(var child in node.children) {
if(node.children.hasOwnProperty(child)) {
queue.push(node.children[child]);
}
}
}
return counter;
};
Trie.prototype.getWords = function() {
var words = [];
var word = '';
this._getWords(this.root, words, words, word);
return words;
};
Trie.prototype._getWords = function(node, words, word) {
for(var child in node.children) {
if(node.children.hasOwnProperty(child)) {
word += child;
if (node.children[child].isWord) {
words.push(word);
}
this._getWords(node.children[child], words, word);
word = word.substring(0, word.length - 1);
}
}
};
Trie.prototype.print = function() {
if(!this.root) {
return console.log('No root node found');
}
var newline = new Node('|');
var queue = [this.root, newline];
var string = '';
while(queue.length) {
var node = queue.shift();
string += node.data.toString() + ' ';
if(node === newline && queue.length) {
queue.push(newline);
}
for(var child in node.children) {
if(node.children.hasOwnProperty(child)) {
queue.push(node.children[child]);
}
}
}
console.log(string.slice(0, -2).trim());
};
Trie.prototype.printByLevel = function() {
if(!this.root) {
return console.log('No root node found');
}
var newline = new Node('\n');
var queue = [this.root, newline];
var string = '';
while(queue.length) {
var node = queue.shift();
string += node.data.toString() + (node.data !== '\n' ? ' ' : '');
if(node === newline && queue.length) {
queue.push(newline);
}
for(var child in node.children) {
if(node.children.hasOwnProperty(child)) {
queue.push(node.children[child]);
}
}
}
console.log(string.trim());
};

var trie = new Trie();