Prefix Search Using Trie in DSA Typescript - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Prefix Search Result

What is the output of the following TypeScript code that inserts words into a Trie and searches for words starting with a prefix?

DSA Typescript

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

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

  insert(word: string): void {
    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.isEndOfWord = true;
  }

  searchPrefix(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 = (currentNode: TrieNode, path: string) => {
      if (currentNode.isEndOfWord) {
        results.push(path);
      }
      for (const [char, childNode] of currentNode.children) {
        dfs(childNode, path + char);
      }
    };
    dfs(node, prefix);
    return results;
  }
}

const trie = new Trie();
trie.insert("apple");
trie.insert("app");
trie.insert("apex");
trie.insert("bat");
trie.insert("bath");
const result = trie.searchPrefix("ap");
console.log(result);

A ["app", "apple", "apex"]

B ["apple", "app", "apex"]

C ["apex", "app", "apple"]

D ["app", "apex"]

💡 Hint

Think about how the DFS collects words in alphabetical order based on children map iteration.

The DFS visits children in insertion order of Map keys, which in JavaScript/TypeScript preserves insertion order. Since 'app' was inserted before 'apple' and 'apex', the results array collects words in the order: 'app', 'apple', 'apex'.

🧠 Conceptual

intermediate

Number of Words with Given Prefix

If a Trie contains the words ["cat", "car", "cart", "dog", "dove"], how many words start with the prefix "car"?

A 0

B 2

C 3

D 1

💡 Hint

Count all words that begin exactly with 'car'.

The words starting with 'car' are 'car' and 'cart', so there are 2 words.

🔧 Debug

advanced

Identify the Bug in Trie Insertion

What error will occur when running this TypeScript code snippet for inserting a word into a Trie?

DSA Typescript

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

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

  insert(word: string): void {
    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); // Bug here
    }
    node.isEndOfWord = true;
  }
}

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

A No error, runs correctly

B SyntaxError: Unexpected token

C ReferenceError: node is not defined

D TypeError: Cannot read property 'isEndOfWord' of undefined

💡 Hint

Check the return type of Map.get and how it's used.

Map.get returns TrieNode | undefined. Without the non-null assertion operator (!), node can become undefined, causing a TypeError when accessing isEndOfWord.

📝 Syntax

advanced

Correct Syntax for DFS in Trie Search

Which option correctly defines the DFS function inside the searchPrefix method to collect all words starting with a prefix?

DSA Typescript

dfs = (currentNode: TrieNode, path: string) => {
  if (currentNode.isEndOfWord) {
    results.push(path);
  }
  for (const [char, childNode] of currentNode.children) {
    dfs(childNode, path + char);
  }
};

A

function dfs(currentNode: TrieNode, path: string) {
  if (currentNode.isEndOfWord) {
    results.push(path);
  }
  for (const [char, childNode] of currentNode.children) {
    dfs(childNode, path + char);
  }
}

B

dfs(currentNode: TrieNode, path: string) => {
  if (currentNode.isEndOfWord) {
    results.push(path);
  }
  for (const [char, childNode] of currentNode.children) {
    dfs(childNode, path + char);
  }
};

C

const dfs = (currentNode: TrieNode, path: string) => {
  if (currentNode.isEndOfWord) {
    results.push(path);
  }
  for (const [char, childNode] of currentNode.children) {
    dfs(childNode, path + char);
  }
};

D

const dfs = function(currentNode: TrieNode, path: string) {
  if (currentNode.isEndOfWord) {
    results.push(path);
  }
  for (const [char, childNode] of currentNode.children) {
    dfs(childNode, path + char);
  }
};

💡 Hint

Arrow functions with const are common for nested functions in TypeScript.

Option C correctly uses a const arrow function with proper syntax. Option C is valid but uses function declaration, which is also correct but less common inside methods. Option C is invalid syntax. Option C is valid but less idiomatic than A.

🚀 Application

expert

Find All Words with Prefix and Sort Alphabetically

Given a Trie with words inserted in any order, which code modification ensures that searchPrefix returns all words starting with the prefix sorted alphabetically?

DSA Typescript

searchPrefix(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 = (currentNode: TrieNode, path: string) => {
    if (currentNode.isEndOfWord) {
      results.push(path);
    }
    for (const [char, childNode] of currentNode.children) {
      dfs(childNode, path + char);
    }
  };
  dfs(node, prefix);
  return results;
}

A Modify DFS to visit children in alphabetical order by sorting keys before recursion.

B Change Map to a JavaScript object and iterate keys sorted alphabetically.

C Use a PriorityQueue to store words during DFS and dequeue all at the end.

D Sort the results array before returning: return results.sort();

💡 Hint

Think about controlling the order of traversal rather than sorting after collection.

Sorting keys before recursion ensures DFS visits children in alphabetical order, so results are collected sorted without extra sorting step. Option A sorts after collection but is less efficient. Option A changes data structure but does not guarantee order. Option A adds complexity.

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