Autocomplete System with Trie in DSA Typescript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

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

DSA Typescript

class TrieNode {
  children: Map<string, TrieNode> = new Map();
  isEndOfWord: boolean = false;
}

const node = new TrieNode();
node.children.[1]('a', new TrieNode());

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

Ainsert

Badd

Cpush

Dset

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'add' which is for Set, not Map.

Using 'push' which is for arrays.

Using 'insert' which is not a Map method.

The Map object uses the 'set' method to add a key-value pair. Here, 'a' is the key and a new TrieNode is the value.

2fill in blank

medium

Complete the code to check if a character exists in the current Trie node's children.

DSA Typescript

function hasChild(node: TrieNode, char: string): boolean {
  return node.children.[1](char);
}

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

Ahas

Bcontains

Cincludes

Dexists

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'contains' which is not a Map method.

Using 'includes' which is for arrays or strings.

Using 'exists' which is not a Map method.

The Map object uses the 'has' method to check if a key exists.

3fill in blank

hard

Fix the error in the insert method to correctly add characters to the Trie.

DSA Typescript

class Trie {
  root: TrieNode = new TrieNode();

  insert(word: string): void {
    let current = this.root;
    for (const char of word) {
      if (!current.children.[1](char)) {
        current.children.set(char, new TrieNode());
      }
      current = current.children.get(char)!;
    }
    current.isEndOfWord = true;
  }
}

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

Ahas

Bget

Cset

Dadd

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'get' which returns a value, not a boolean.

Using 'set' which adds or updates but does not check existence.

Using 'add' which is not a Map method.

We must check if the character exists using 'has' before adding it with 'set'.

4fill in blank

hard

Fill both blanks to complete the search method that returns true if the word exists in the Trie.

DSA Typescript

search(word: string): boolean {
  let current = this.root;
  for (const char of word) {
    if (!current.children.[1](char)) {
      return false;
    }
    current = current.children.[2](char)!;
  }
  return current.isEndOfWord;
}

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

Ahas

Bget

Cset

Dadd

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'set' instead of 'get' to retrieve the child node.

Using 'add' which is not a Map method.

Using 'get' to check existence which returns undefined if missing.

Use 'has' to check if the character exists and 'get' to move to the child node.

5fill in blank

hard

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

DSA Typescript

autocomplete(prefix: string): string[] {
  let current = this.root;
  for (const char of prefix) {
    if (!current.children.[1](char)) {
      return [];
    }
    current = current.children.[2](char)!;
  }
  const results: string[] = [];
  function dfs(node: TrieNode, path: string) {
    if (node.isEndOfWord) {
      results.push(path);
    }
    for (const [nextChar, nextNode] of node.children) {
      dfs(nextNode, path + [3]);
    }
  }
  dfs(current, prefix);
  return results;
}

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

Ahas

Bget

CnextChar

Dchar

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'char' instead of 'nextChar' inside dfs loop.

Using 'set' or 'add' instead of 'has' or 'get'.

Not adding the character to the path string.

Use 'has' to check child existence, 'get' to get the child node, and 'nextChar' to add the next character to the path.

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 fast prefix-based searches.

2. Step 2: Connect Trie usage to autocomplete

   Autocomplete requires quick lookup of words starting with a given prefix, which Trie supports efficiently.

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-based fast search [OK]

Common Mistakes:

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

2. Which of the following is the correct TypeScript syntax to define a Trie node with children as a Map of characters to Trie nodes?

easy

A. class TrieNode { children: Array<string> = []; isEnd: boolean = false; }

B. class TrieNode { children: Map = new Map(); isEnd: boolean = false; }

C. class TrieNode { children: string[] = []; isEnd: boolean = true; }

D. class TrieNode { children: Map = new Map(); isEnd: boolean = false; }

Solution

1. Step 1: Identify correct data type for children

   Children should map characters (string) to TrieNode objects, so Map is correct.

2. Step 2: Check initialization and isEnd flag

   Children initialized as new Map(), and isEnd defaults to false indicating not end of word initially.

3. Final Answer:

   class TrieNode { children: Map = new Map(); isEnd: boolean = false; } -> Option B

4. Quick Check:

   Map for children [OK]

Hint: Children map chars to nodes using Map [OK]

Common Mistakes:

• Using array instead of Map for children
• Using number keys instead of string keys
• Setting isEnd to true by default

3. Given the following TypeScript code snippet for inserting words into a Trie and searching for prefix suggestions, what will be the output of autocomplete.getSuggestions('ca')?

class TrieNode {
  children: Map = new Map();
  isEnd: boolean = false;
}

class AutocompleteSystem {
  root: TrieNode = new TrieNode();

  insert(word: string) {
    let node = this.root;
    for (const char of word) {
      if (!node.children.has(char)) {
        node.children.set(char, new TrieNode());
      }
      node = node.children.get(char)!;
    }
    node.isEnd = true;
  }

  getSuggestions(prefix: string): string[] {
    let node = this.root;
    for (const char of prefix) {
      if (!node.children.has(char)) return [];
      node = node.children.get(char)!;
    }
    const results: string[] = [];
    const dfs = (currNode: TrieNode, path: string) => {
      if (currNode.isEnd) results.push(path);
      for (const [ch, nextNode] of currNode.children) {
        dfs(nextNode, path + ch);
      }
    };
    dfs(node, prefix);
    return results;
  }
}

const autocomplete = new AutocompleteSystem();
autocomplete.insert('cat');
autocomplete.insert('car');
autocomplete.insert('cap');
autocomplete.insert('dog');
console.log(autocomplete.getSuggestions('ca'));

medium

A. ["cat", "car"]

B. ["dog"]

C. []

D. ["cat", "car", "cap"]

Solution

1. Step 1: Insert words and traverse prefix 'ca'

   Words 'cat', 'car', 'cap' are inserted. Searching prefix 'ca' finds node with children 't', 'r', 'p'.

2. Step 2: DFS collects all words starting with 'ca'

   DFS from 'ca' node collects 'cat', 'car', and 'cap' as all end nodes under that prefix.

3. Final Answer:

   ["cat", "car", "cap"] -> Option D

4. Quick Check:

   Prefix 'ca' matches cat, car, cap [OK]

Hint: DFS from prefix node collects all words [OK]

Common Mistakes:

• Returning empty list if prefix found
• Returning unrelated words like 'dog'
• Missing one or more words starting with prefix

4. The following TypeScript code for the insert method in a Trie has a bug. What is the bug?

insert(word: string) {
  let node = this.root;
  for (const char of word) {
    if (!node.children.has(char)) {
      node.children.set(char, new TrieNode());
    }
    node = node.children.get(char);
  }
  node.isEnd = true;
}

medium

A. Missing non-null assertion operator (!) when getting child node

B. Children map should be an array, not Map

C. isEnd should be set to false, not true

D. Loop should iterate over indices, not characters

Solution

1. Step 1: Identify potential null or undefined access

   node.children.get(char) can return undefined if key missing, so TypeScript requires non-null assertion or check.

2. Step 2: Fix by adding non-null assertion operator

   Adding '!' after get(char) tells TypeScript the value is not undefined, fixing the bug.

3. Final Answer:

   Missing non-null assertion operator (!) when getting child node -> Option A

4. Quick Check:

   Use get(char)! to avoid undefined [OK]

Hint: Use '!' after get() to assure non-null [OK]

Common Mistakes:

• Ignoring TypeScript undefined error
• Changing data structure unnecessarily
• Setting isEnd incorrectly

5. You want to extend the autocomplete system to return suggestions sorted by their frequency of insertion (most frequent first). Which data structure modification is best to support this efficiently?

hard

A. Store all words in a separate array and sort on each query

B. Use a binary search tree instead of a Trie

C. Add a frequency count field to each Trie node marking end of word

D. Remove the isEnd flag and rely on children count

Solution

1. Step 1: Understand frequency tracking in Trie

   Adding a frequency count to nodes marking word ends allows tracking how often each word was inserted.

2. Step 2: Use frequency to sort suggestions efficiently

   During DFS for suggestions, use frequency counts to sort or prioritize words without extra data structures.

3. Final Answer:

   Add a frequency count field to each Trie node marking end of word -> Option C

4. Quick Check:

   Frequency field in Trie nodes enables sorting [OK]

Hint: Track frequency in nodes to sort suggestions fast [OK]

Common Mistakes:

• Sorting all words on each query (slow)
• Replacing Trie with BST loses prefix efficiency
• Removing isEnd loses word boundary info