Autocomplete System with Trie in DSA Javascript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a new Trie node with an empty children object.

DSA Javascript

class TrieNode {
  constructor() {
    this.children = [1];
    this.isEndOfWord = false;
  }
}

Drag options to blanks, or click blank then click option'

A{}

Bnull

C[]

D''

Attempts:

3 left

💡 Hint

Common Mistakes

Using an array [] instead of an object for children.

Setting children to null or empty string.

The children of a Trie node should be an empty object to hold child nodes by character keys.

2fill in blank

medium

Complete the code to insert a character into the Trie children if it does not exist.

DSA Javascript

insert(word) {
  let node = this.root;
  for (let char of word) {
    if (!node.children.hasOwnProperty([1])) {
      node.children[char] = new TrieNode();
    }
    node = node.children[char];
  }
  node.isEndOfWord = true;
}

Drag options to blanks, or click blank then click option'

Achar

Bchildren

Cnode

Dword

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'word' or 'node' instead of 'char' as the key.

Checking children without using hasOwnProperty.

We check if the current character 'char' exists as a key in the children object.

3fill in blank

hard

Fix the error in the search method to correctly check if a word exists in the Trie.

DSA Javascript

search(word) {
  let node = this.root;
  for (let char of word) {
    if (!node.children[[1]]) {
      return false;
    }
    node = node.children[char];
  }
  return node.isEndOfWord;
}

Drag options to blanks, or click blank then click option'

Anode

Bword

Cchar

Dchildren

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'word' or 'node' instead of 'char' as the key.

Checking children without proper key.

We must check if the current character 'char' exists in the children object.

4fill in blank

hard

Fill both blanks to complete the DFS helper function that collects all words starting from a node.

DSA Javascript

dfs(node, prefix, results) {
  if (node.isEndOfWord) {
    results.push([1]);
  }
  for (let char in node.children) {
    this.dfs(node.children[char], prefix [2] char, results);
  }
}

Drag options to blanks, or click blank then click option'

Aprefix

Bprefix + char

C+

D-

Attempts:

3 left

💡 Hint

Common Mistakes

Pushing the wrong variable instead of prefix.

Using '-' instead of '+' for string concatenation.

Push the current prefix as a word when isEndOfWord is true, and concatenate prefix and char with '+' when recursing.

5fill in blank

hard

Fill the blanks to complete the autocomplete method that returns all words starting with a prefix.

DSA Javascript

autocomplete(prefix) {
  let node = this.root;
  for (let char of prefix) {
    if (!node.children[[1]]) {
      return [];
    }
    node = node.children[char];
  }
  let results = [];
  this.dfs(node, [2], results);
  return results;
}

Drag options to blanks, or click blank then click option'

Aprefix

Bchar

D''

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'prefix' instead of 'char' in the loop check.

Passing empty string instead of prefix to dfs.

Check if the current character 'char' exists in children, then call dfs with the full prefix string to collect words.

Practice

1. What is the main purpose of using a Trie in an autocomplete system?

easy

A. To efficiently store and search prefixes of words

B. To sort words alphabetically

C. To count the frequency of words

D. To encrypt the stored words

Solution

1. Step 1: Understand the role of Trie in prefix search

   A Trie is a tree-like data structure designed to store strings by their prefixes, allowing quick prefix-based searches.

2. Step 2: Relate Trie to autocomplete functionality

   Autocomplete needs to find all words starting with a given prefix efficiently, which Trie supports by traversing nodes matching the prefix.

3. Final Answer:

   To efficiently store and search prefixes of words -> Option A

4. Quick Check:

   Trie = prefix search [OK]

Hint: Trie is built for prefix matching, perfect for autocomplete [OK]

Common Mistakes:

• Confusing Trie with sorting data structure
• Thinking Trie encrypts data
• Assuming Trie counts word frequency only

2. Which of the following is the correct way to insert a word "cat" into a Trie node in JavaScript?

easy

A. node.children.push('c');

B. node.children['c'] = new TrieNode();

C. node['c'] = 'cat';

D. node.add('cat');

Solution

1. Step 1: Understand Trie node structure

   Each Trie node has a children object mapping characters to child nodes, so to insert 'c', we assign a new TrieNode to node.children['c'].

2. Step 2: Check each option's correctness

   node.children['c'] = new TrieNode(); correctly assigns a new TrieNode to the character key. Options B, C, and D do not follow the standard Trie insertion method.

3. Final Answer:

   node.children['c'] = new TrieNode(); -> Option B

4. Quick Check:

   Insert char by assigning new TrieNode in children map [OK]

Hint: Use children map with character keys for Trie insertion [OK]

Common Mistakes:

• Using array push instead of map assignment
• Assigning string instead of TrieNode
• Calling undefined add method

3. Given the following code snippet for searching words with prefix "app" in a Trie, what will be the output?

const trie = new Trie();
trie.insert("apple");
trie.insert("app");
trie.insert("application");
const results = trie.searchPrefix("app");
console.log(results);

medium

A. ["apple", "application"]

B. ["app"]

C. []

D. ["apple", "app", "application"]

Solution

1. Step 1: Insert words into Trie

   Words "apple", "app", and "application" are inserted, so all prefixes starting with "app" exist in the Trie.

2. Step 2: Search for prefix "app"

   The searchPrefix method returns all words starting with "app", which includes all three inserted words.

3. Final Answer:

   ["apple", "app", "application"] -> Option D

4. Quick Check:

   All words with prefix "app" returned [OK]

Hint: Search prefix returns all words starting with it [OK]

Common Mistakes:

• Expecting only exact matches
• Ignoring shorter word "app" itself
• Assuming empty result if prefix is a word

4. The following code tries to insert words into a Trie but throws an error. What is the bug?

class TrieNode {
  constructor() {
    this.children = [];
    this.isEnd = false;
  }
}

class Trie {
  constructor() {
    this.root = new TrieNode();
  }
  insert(word) {
    let node = this.root;
    for (let char of word) {
      if (!node.children[char]) {
        node.children[char] = new TrieNode();
      }
      node = node.children[char];
    }
    node.isEnd = true;
  }
}

const trie = new Trie();
trie.insert("dog");

medium

A. children should be an object, not an array

B. isEnd should be initialized to true

C. Missing return statement in insert method

D. for loop should use index instead of for-of

Solution

1. Step 1: Analyze children data structure

   children is initialized as an empty array, but characters are used as keys, which are strings, not numeric indices.

2. Step 2: Identify correct data structure for children

   children should be an object (map) to use string keys like 'd', 'o', 'g'. Using array causes undefined behavior and errors.

3. Final Answer:

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

4. Quick Check:

   Use object for children keys [OK]

Hint: Use object for children to map chars, not array [OK]

Common Mistakes:

• Using array for children with string keys
• Setting isEnd incorrectly
• Thinking loop type causes error

5. You want to build an autocomplete system that returns the top 2 most frequent words starting with a prefix. Which modification to a Trie node helps achieve this efficiently?

hard

A. Use an array instead of object for children

B. Store only the full words at leaf nodes

C. Store a frequency count and maintain a sorted list of top words at each node

D. Store the entire dictionary at each node

Solution

1. Step 1: Understand requirement for top frequent words

   To quickly return top 2 frequent words for any prefix, each node should track frequency info and top words under it.

2. Step 2: Choose data structure modification

   Storing frequency counts and maintaining a sorted list of top words at each node allows fast retrieval without full traversal.

3. Final Answer:

   Store a frequency count and maintain a sorted list of top words at each node -> Option C

4. Quick Check:

   Frequency + top words per node = efficient autocomplete [OK]

Hint: Keep top words and frequencies at nodes for fast autocomplete [OK]

Common Mistakes:

• Storing full dictionary at each node wastes memory
• Using arrays for children slows lookup
• Ignoring frequency info for ranking