Trie Node Design and Initialization in DSA Javascript

Mental Model

A trie node holds links to next letters and a marker if a word ends here.

Analogy: Think of a trie node like a mailbox with many slots, one for each letter, and a flag to say if a full word stops here.

root
 ↓
[ ] -> [ ] -> [ ] -> ... -> [ ]
 ↑
Each box is a letter slot (child), empty at start, with a flag for word end.

Dry Run Walkthrough

Input: Initialize a trie node with 3 possible letters: a, b, c

Goal: Create a node that can hold children for letters a, b, c and mark if a word ends here

Step 1: Create an empty children object to hold letter links

Node: children = {}, isEndOfWord = false

Why: We need a place to store links to next letters

Step 2: Set isEndOfWord flag to false initially

Node: children = {}, isEndOfWord = false

Why: No word ends here yet, so flag is false

Result:

Node: children = {}, isEndOfWord = false

Annotated Code

DSA Javascript

class TrieNode {
  constructor() {
    this.children = {} // holds links to next letters
    this.isEndOfWord = false // marks if a word ends here
  }
}

// Driver code to create a node
const node = new TrieNode()
console.log(node)

this.children = {} // holds links to next letters

initialize empty container for child nodes

this.isEndOfWord = false // marks if a word ends here

initialize word end marker as false

Output

TrieNode { children: {}, isEndOfWord: false }

Complexity Analysis

Time: O(1) because creating a node just sets up empty properties

Space: O(1) for the node itself; children grow as words are added

vs Alternative: Compared to arrays for children, using an object saves space when few children exist

Edge Cases

Node with no children and no word end

Node correctly shows empty children and isEndOfWord false

DSA Javascript

this.children = {} // holds links to next letters

When to Use This Pattern

When you need to store many words sharing prefixes, use trie nodes with children and end markers to build the trie structure.

Common Mistakes

Mistake: Not initializing children as an empty object or array
Fix: Always initialize children container in constructor to avoid errors when adding letters

Mistake: Forgetting to set isEndOfWord flag to false initially
Fix: Set isEndOfWord to false in constructor to mark no word ends at new node

Summary

It creates a trie node with empty children and a word end flag.

Use it when building tries to store words by letters.

Remember: children hold next letters, isEndOfWord marks word completion.

Practice

1. What is the main purpose of a Trie node in a Trie data structure?

easy

A. To store the entire word in one node

B. To store integer values for sorting

C. To hold only the last character of a word

D. To store characters and links to child nodes representing prefixes

Solution

1. Step 1: Understand Trie node role

   A Trie node stores a character and links to child nodes that represent possible next characters in words.

2. Step 2: Compare options

   Only To store characters and links to child nodes representing prefixes correctly describes storing characters and child links for prefixes.

3. Final Answer:

   To store characters and links to child nodes representing prefixes -> Option D

4. Quick Check:

   Trie node = character + child links [OK]

Hint: Trie nodes hold characters and children, not full words [OK]

Common Mistakes:

• Thinking nodes store whole words
• Confusing Trie nodes with array elements
• Assuming nodes only store last characters

2. Which of the following is the correct way to initialize a Trie node in JavaScript with an empty children object and a boolean flag isEndOfWord set to false?

easy

A. const node = { children: null, isEndOfWord: true };

B. const node = { children: [], isEndOfWord: false };

C. const node = { children: {}, isEndOfWord: false };

D. const node = { children: {}, isEndOfWord: true };

Solution

1. Step 1: Identify correct children type

   Children should be an object to map characters to nodes, so {} is correct.

2. Step 2: Check isEndOfWord initial value

   Initially, isEndOfWord should be false because the node does not end a word yet.

3. Final Answer:

   const node = { children: {}, isEndOfWord: false }; -> Option C

4. Quick Check:

   Empty object + false flag = correct initialization [OK]

Hint: Children is an object, isEndOfWord starts false [OK]

Common Mistakes:

• Using array instead of object for children
• Setting isEndOfWord true at initialization
• Setting children to null

3. Consider this JavaScript code snippet for a Trie node:

class TrieNode {
  constructor() {
    this.children = {};
    this.isEndOfWord = false;
  }
}

const node = new TrieNode();
console.log(node.isEndOfWord);
console.log(Object.keys(node.children).length);

What will be the output?

medium

A. false
0

B. true
0

C. false
undefined

D. undefined
0

Solution

1. Step 1: Check isEndOfWord value after construction

   Constructor sets isEndOfWord to false, so console.log prints false.

2. Step 2: Check children keys count

   Children is an empty object, so Object.keys(node.children).length is 0.

3. Final Answer:

   false 0 -> Option A

4. Quick Check:

   isEndOfWord = false, children keys = 0 [OK]

Hint: Empty children means zero keys; isEndOfWord defaults false [OK]

Common Mistakes:

• Assuming isEndOfWord is true by default
• Expecting children keys length to be undefined
• Confusing object keys count with children length

4. The following code attempts to create a Trie node but has an error:

function TrieNode() {
  this.children = [];
  this.isEndOfWord = false;
}

What is the main problem with this code?

medium

A. children should be an object, not an array

B. isEndOfWord should be true initially

C. Function should return a value

D. Missing semicolon after isEndOfWord assignment

Solution

1. Step 1: Analyze children data type

   Children must be an object to map characters to nodes, not an array.

2. Step 2: Check other parts

   isEndOfWord false is correct; function constructors do not need explicit return; semicolons are optional in JS.

3. Final Answer:

   children should be an object, not an array -> Option A

4. Quick Check:

   Children = object, not array [OK]

Hint: Children must be object for character keys, not array [OK]

Common Mistakes:

• Thinking arrays can map characters
• Expecting function constructors to return explicitly
• Focusing on semicolon errors

5. You want to design a Trie node class in JavaScript that supports Unicode characters as keys in children and initializes isEndOfWord to false. Which of the following implementations correctly achieves this and allows adding child nodes dynamically?

hard

A. class TrieNode {
constructor() {
this.children = {};
this.isEndOfWord = true;
}
addChild(char) {
this.children[char] = new TrieNode();
}
}

B. class TrieNode {
constructor() {
this.children = new Map();
this.isEndOfWord = false;
}
addChild(char) {
if (!this.children.has(char)) {
this.children.set(char, new TrieNode());
}
}
}

C. class TrieNode {
constructor() {
this.children = [];
this.isEndOfWord = false;
}
addChild(char) {
this.children.push(char);
}
}

D. class TrieNode {
constructor() {
this.children = new Set();
this.isEndOfWord = false;
}
addChild(char) {
this.children.add(char);
}
}

Solution

1. Step 1: Check children data structure for Unicode support

   Using Map allows any string keys including Unicode, better than plain objects.

2. Step 2: Verify isEndOfWord initialization and addChild method

   isEndOfWord is false initially, and addChild adds new TrieNode only if missing, correctly updating children Map.

3. Step 3: Compare other options

   class TrieNode { constructor() { this.children = {}; this.isEndOfWord = true; } addChild(char) { this.children[char] = new TrieNode(); } } sets isEndOfWord true initially (wrong). class TrieNode { constructor() { this.children = []; this.isEndOfWord = false; } addChild(char) { this.children.push(char); } } uses array which can't map chars to nodes. class TrieNode { constructor() { this.children = new Set(); this.isEndOfWord = false; } addChild(char) { this.children.add(char); } } uses Set which stores chars but not nodes.

4. Final Answer:

   Implementation using Map with isEndOfWord false and addChild method -> Option B

5. Quick Check:

   Map + false flag + dynamic addChild = correct Trie node [OK]

Hint: Use Map for Unicode keys and false isEndOfWord [OK]

Common Mistakes:

• Using array or Set instead of Map or object
• Setting isEndOfWord true at start
• Not checking if child exists before adding