Prefix Search Using Trie in DSA Typescript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to insert a word into the Trie.

DSA Typescript

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[1];
  }
  node.isEndOfWord = true;
}

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

A.children.delete(char)

B.children.set(char)

C.children.has(char)

D.children.get(char)

Attempts:

3 left

💡 Hint

Common Mistakes

Using set instead of get to move to the child node.

Trying to check if the child exists instead of moving to it.

We move to the child node corresponding to the current character using node.children.get(char).

2fill in blank

medium

Complete the code to check if a prefix exists in the Trie.

DSA Typescript

startsWith(prefix: string): boolean {
  let node = this.root;
  for (const char of prefix) {
    if (!node.children.has(char)) {
      return false;
    }
    node = node[1];
  }
  return true;
}

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

A.children.set(char)

B.children.get(char)

C.children.has(char)

D.children.delete(char)

Attempts:

3 left

💡 Hint

Common Mistakes

Using set instead of get to move to the child node.

Checking for existence but not moving to the child node.

To traverse the Trie for each character in the prefix, we use node.children.get(char).

3fill in blank

hard

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

DSA Typescript

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

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

A.children.get(char)

B.children.set(char)

C.children.has(char)

D.children.delete(char)

Attempts:

3 left

💡 Hint

Common Mistakes

Using set instead of get to move to the child node.

Not moving to the child node after checking existence.

We must move to the child node for each character using node.children.get(char) to check the word properly.

4fill in blank

hard

Complete the code to implement a method that collects all words starting from a given node.

DSA Typescript

private collectWords(node: TrieNode, prefix: string, words: string[]) {
  if (node.isEndOfWord) {
    words.push(prefix);
  }
  for (const [char, childNode] of node.children) {
    this.collectWords(childNode, prefix[1] char, words,);
  }
}

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

A+

B,

C.

D-

Attempts:

3 left

💡 Hint

Common Mistakes

Using '.' instead of '+' to concatenate strings.

Using '.' instead of ',' to separate function arguments.

We concatenate the character to the prefix using '+' and pass the words array as the second argument separated by a comma.

5fill in blank

hard

Fill both blanks to implement the prefix search method that returns all words starting with the given prefix.

DSA Typescript

prefixSearch(prefix: string): string[] {
  let node = this.root;
  for (const char of prefix) {
    if (!node.children.has(char)) {
      return [];
    }
    node = node[1];
  }
  const words: string[] = [];
  this.collectWords(node, prefix[2], words,);
  return words;
}

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

A.children.get(char)

B+

C,

D.children.has(char)

Attempts:

3 left

💡 Hint

Common Mistakes

Using .children.has(char) instead of .children.get(char) to move nodes.

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

Using '.' instead of ',' to separate function arguments.

We move to the child node with .children.get(char), concatenate prefix and call collectWords with a comma-separated words array.

Practice

1. What is the main purpose of using a Trie data structure in prefix search?

easy

A. To efficiently find all words that start with a given prefix

B. To sort words alphabetically

C. To store words in reverse order

D. To count the total number of words

Solution

1. Step 1: Understand Trie usage

   A Trie stores words by their prefixes, allowing quick prefix-based lookups.

2. Step 2: Identify prefix search goal

   Prefix search means finding all words starting with a given prefix efficiently.

3. Final Answer:

   To efficiently find all words that start with a given prefix -> Option A

4. Quick Check:

   Trie = Prefix search [OK]

Hint: Trie is built for prefix-based fast word retrieval [OK]

Common Mistakes:

• Confusing Trie with sorting structure
• Thinking Trie stores words reversed
• Assuming Trie counts total words only

2. Which of the following is the correct TypeScript syntax to define a Trie node with children as a Map and an endOfWord boolean?

easy

A. class TrieNode { children: object; endOfWord: string; constructor() { this.children = {}; this.endOfWord = ''; } }

B. class TrieNode { children: string[]; endOfWord: boolean; constructor() { this.children = []; this.endOfWord = false; } }

C. class TrieNode { children: Map; endOfWord: boolean; constructor() { this.children = new Map(); this.endOfWord = false; } }

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

Solution

1. Step 1: Check children type

   Children should be a Map with string keys and TrieNode values for prefix characters.

2. Step 2: Check endOfWord type

   endOfWord is a boolean indicating if node ends a word.

3. Final Answer:

   class TrieNode { children: Map; endOfWord: boolean; constructor() { this.children = new Map(); this.endOfWord = false; } } -> Option D

4. Quick Check:

   Map + boolean endOfWord = class TrieNode { children: Map; endOfWord: boolean; constructor() { this.children = new Map(); this.endOfWord = false; } } [OK]

Hint: Use Map for children and boolean for endOfWord [OK]

Common Mistakes:

• Using array instead of Map for children
• Using wrong key type in Map
• Setting endOfWord as string instead of boolean

3. Given the following TypeScript code snippet, what will be the output of console.log(trie.startsWith('app'))?

class TrieNode {
  children = new Map();
  endOfWord = false;
}

class Trie {
  root = 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.endOfWord = true;
  }

  startsWith(prefix: string): boolean {
    let node = this.root;
    for (const char of prefix) {
      if (!node.children.has(char)) return false;
      node = node.children.get(char)!;
    }
    return true;
  }
}

const trie = new Trie();
trie.insert('apple');
trie.insert('app');
trie.insert('bat');

medium

A. true

B. false

C. undefined

D. Error at runtime

Solution

1. Step 1: Insert words into Trie

   Words 'apple', 'app', and 'bat' are inserted, so prefix 'app' exists in Trie.

2. Step 2: Check startsWith method

   Method returns true if prefix characters exist in Trie nodes, which they do for 'app'.

3. Final Answer:

   true -> Option A

4. Quick Check:

   Prefix 'app' found = true [OK]

Hint: If prefix inserted, startsWith returns true [OK]

Common Mistakes:

• Confusing startsWith with search for whole word
• Expecting false if prefix is a word itself
• Thinking startsWith returns endOfWord status

4. The following TypeScript code tries to implement a startsWith method for a Trie but always returns false. What is the bug?

startsWith(prefix: string): boolean {
  let node = this.root;
  for (const char of prefix) {
    if (!node.children.has(char)) return false;
    node = node.children.get(char);
  }
  return false;
}

medium

A. It should check endOfWord before returning

B. It should use a for loop with index instead of for-of

C. It should return true after the loop if prefix exists

D. It should initialize node with a new TrieNode

Solution

1. Step 1: Analyze loop behavior

   The loop correctly traverses nodes for each prefix character.

2. Step 2: Check return statement

   Method returns false always after loop, so never returns true even if prefix found.

3. Step 3: Fix return logic

   Return true after loop if all prefix chars found.

4. Final Answer:

   It should return true after the loop if prefix exists -> Option C

5. Quick Check:

   Return true after loop if prefix found [OK]

Hint: Return true after loop if prefix nodes exist [OK]

Common Mistakes:

• Returning false at end regardless
• Checking endOfWord unnecessarily
• Incorrect node initialization

5. You want to extend a Trie to return all words starting with a given prefix. Which approach correctly implements this in TypeScript? A) Traverse to prefix node, then recursively collect all words below. B) Store all words in an array and filter by prefix each time. C) Use a hash map to map prefixes to words directly. D) Insert reversed words and search suffixes instead.

hard

A. Store all words in an array and filter by prefix each time.

B. Traverse to prefix node, then recursively collect all words below.

C. Use a hash map to map prefixes to words directly.

D. Insert reversed words and search suffixes instead.

Solution

1. Step 1: Understand prefix search extension

   To get all words with prefix, first find node matching prefix.

2. Step 2: Collect words recursively

   From prefix node, recursively traverse children to collect all complete words.

3. Step 3: Evaluate other options

   Filtering array or hash map is less efficient; reversed words search suffixes, not prefixes.

4. Final Answer:

   Traverse to prefix node, then recursively collect all words below. -> Option B

5. Quick Check:

   Recursive traversal from prefix node = Traverse to prefix node, then recursively collect all words below. [OK]

Hint: Find prefix node, then DFS to collect words [OK]

Common Mistakes:

• Filtering all words each time (slow)
• Using hash map without prefix traversal
• Confusing prefix with suffix search