Trie vs Hash Map for Prefix Matching in DSA Javascript - Interactive Comparison Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to insert a word into the Trie.

DSA Javascript

function insertWord(root, word) {
  let node = root;
  for (let char of word) {
    if (!node.children.has(char)) {
      node.children.set(char, [1]);
    }
    node = node.children.get(char);
  }
  node.isEnd = true;
}

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

A{}

Bnew Set()

C[]

Dnew Map()

Attempts:

3 left

💡 Hint

Common Mistakes

Using an array or object instead of Map for children.

Not initializing children for new nodes.

Each node's children is a Map to store next characters and their nodes. So we create a new Map for a new child.

2fill in blank

medium

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

DSA Javascript

function startsWith(root, prefix) {
  let node = root;
  for (let char of prefix) {
    if (!node.children.has([1])) {
      return false;
    }
    node = node.children.get(char);
  }
  return true;
}

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

Aprefix

Bnode

Cchar

Droot

Attempts:

3 left

💡 Hint

Common Mistakes

Using the whole prefix instead of the current character.

Using the node or root instead of the character.

We check if the current node's children has the current character from the prefix.

3fill in blank

hard

Fix the error in the Hash Map prefix search function to correctly check prefixes.

DSA Javascript

function hashMapStartsWith(map, prefix) {
  for (let key of map.keys()) {
    if (key[1]prefix) {
      return true;
    }
  }
  return false;
}

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

AendsWith(

BstartsWith(

CindexOf(

Dincludes(

Attempts:

3 left

💡 Hint

Common Mistakes

Using includes or indexOf which check anywhere in the string.

Using endsWith which checks the end of the string.

To check if a key starts with the prefix, use the startsWith method.

4fill in blank

hard

Fill both blanks to create a Trie node and mark the end of a word.

DSA Javascript

function createNode() {
  return {
    children: [1],
    isEnd: [2]
  };
}

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

Anew Map()

Bfalse

Ctrue

D{}

Attempts:

3 left

💡 Hint

Common Mistakes

Setting isEnd to true on creation.

Using an object or array instead of Map for children.

A Trie node has children as a Map and isEnd initially false.

5fill in blank

hard

Fill all three blanks to build a Hash Map from words and check prefix existence.

DSA Javascript

function buildHashMap(words) {
  const map = new Map();
  for (const word of words) {
    map.set(word, [1]);
  }
  return map;
}

function checkPrefix(map, prefix) {
  for (const key of map.keys()) {
    if (key.[2](prefix)) {
      return [3];
    }
  }
  return false;
}

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

Atrue

BstartsWith

Dfalse

Attempts:

3 left

💡 Hint

Common Mistakes

Returning false instead of true when prefix matches.

Using wrong string method for prefix check.

We store true for each word in the map. To check prefix, use startsWith and return true if found.

Practice

1. Which data structure is generally more efficient for prefix matching in a large set of strings?

easy

A. Linked List, because it allows sequential access

B. Hash Map, because it stores keys with constant time lookup

C. Array, because it stores elements in order

D. Trie, because it stores prefixes explicitly and allows fast prefix search

Solution

1. Step 1: Understand prefix matching requirement

   Prefix matching means finding all strings that start with a given prefix efficiently.

2. Step 2: Compare Trie and Hash Map for prefix search

   Trie stores prefixes explicitly, allowing quick traversal to all words with that prefix. Hash Map does not store prefixes, so it requires checking all keys or additional structures.

3. Final Answer:

   Trie, because it stores prefixes explicitly and allows fast prefix search -> Option D

4. Quick Check:

   Prefix search = Trie [OK]

Hint: Trie stores prefixes explicitly, ideal for prefix search [OK]

Common Mistakes:

• Thinking hash map can do prefix search directly
• Confusing prefix matching with exact key lookup
• Assuming arrays or linked lists are efficient for prefix search

2. Which of the following is the correct way to check if a prefix exists in a Trie node in JavaScript?

easy

A. if (prefix in trieNode) { ... }

B. if (trieNode.hasOwnProperty(prefix)) { ... }

C. if (trieNode[prefix]) { ... }

D. if (trieNode.contains(prefix)) { ... }

Solution

1. Step 1: Understand how to check keys in JavaScript objects

   The 'in' operator checks if a property exists in an object or its prototype chain.

2. Step 2: Compare options for checking prefix in Trie node

   if (prefix in trieNode) { ... } uses 'prefix in trieNode', which correctly checks if the prefix key exists. if (trieNode.hasOwnProperty(prefix)) { ... } uses hasOwnProperty which is valid but less common for prefix check. if (trieNode[prefix]) { ... } checks truthiness which can fail if value is falsy. if (trieNode.contains(prefix)) { ... } uses a non-existent method.

3. Final Answer:

   if (prefix in trieNode) { ... } -> Option A

4. Quick Check:

   Use 'in' operator to check keys [OK]

Hint: Use 'in' operator to check keys in objects [OK]

Common Mistakes:

• Using non-existent 'contains' method
• Checking property truthiness instead of existence
• Confusing hasOwnProperty with 'in' operator

3. What will be the output of the following code snippet?

const trie = {};
function insert(word) {
  let node = trie;
  for (const char of word) {
    if (!(char in node)) node[char] = {};
    node = node[char];
  }
  node.isEnd = true;
}

insert('cat');
insert('car');
insert('dog');

console.log('c' in trie, 'a' in trie['c'], 't' in trie['c']['a'], trie['c']['a']['t'].isEnd);

medium

A. true true true true

B. true true false undefined

C. true false false undefined

D. false false false undefined

Solution

1. Step 1: Trace insertions of words into the trie

   Inserting 'cat' creates nodes c -> a -> t with isEnd true at 't'. Inserting 'car' adds r under c -> a. Inserting 'dog' adds d -> o -> g separately.

2. Step 2: Check existence of keys and isEnd property

   'c' exists in trie, 'a' exists under 'c', 't' exists under 'a', and 't' node has isEnd true from 'cat' insertion.

3. Final Answer:

   true true true true -> Option A

4. Quick Check:

   All keys and isEnd exist as expected [OK]

Hint: Trace each insertion step-by-step [OK]

Common Mistakes:

• Assuming 't' node missing after insertions
• Confusing isEnd property placement
• Not updating node reference inside loop

4. The following code attempts to check if a prefix exists in a Trie but does not work correctly. What is the bug?

function startsWith(trie, prefix) {
  let node = trie;
  for (const char of prefix) {
    if (!node[char]) return false;
    node = node[char];
  }
  return true;
}

medium

A. It incorrectly returns false if a node exists but is falsy

B. It does not check if the prefix is a complete word

C. It should use 'char in node' instead of '!node[char]'

D. It does not handle empty prefix input

Solution

1. Step 1: Analyze the condition '!node[char]'

   This condition fails if node[char] exists but is falsy (like an empty object or false), causing incorrect false return.

2. Step 2: Suggest correct key existence check

   Using 'char in node' correctly checks if the key exists regardless of value truthiness.

3. Final Answer:

   It should use 'char in node' instead of '!node[char]' -> Option C

4. Quick Check:

   Use 'in' operator to check keys, not truthiness [OK]

Hint: Check key existence with 'in', not truthiness [OK]

Common Mistakes:

• Using falsy check instead of key existence
• Confusing prefix existence with complete word check
• Ignoring empty prefix edge case

5. You want to implement a prefix search that returns all words starting with a given prefix from a large dictionary. Which approach is best and why?

// Approach 1: Use a Hash Map storing all words as keys
// Approach 2: Use a Trie storing words character by character
// Approach 3: Use an array and filter words by prefix
// Approach 4: Use a sorted array and binary search for prefix

hard

A. Approach 4, because binary search on sorted array quickly finds prefix range

B. Approach 2, because Trie allows efficient prefix traversal and retrieval

C. Approach 3, because filtering arrays is simple and fast for large data

D. Approach 1, because Hash Map has constant time lookup for prefixes

Solution

1. Step 1: Evaluate each approach for prefix search

   Hash Map (Approach 1) cannot do prefix search efficiently as keys are exact. Array filter (Approach 3) is slow for large data. Sorted array with binary search (Approach 4) helps but retrieving all matches requires scanning range.

2. Step 2: Understand Trie advantages

   Trie (Approach 2) stores prefixes explicitly, allowing fast traversal to all words sharing the prefix, making it best for prefix search in large datasets.

3. Final Answer:

   Approach 2, because Trie allows efficient prefix traversal and retrieval -> Option B

4. Quick Check:

   Prefix search efficiency = Trie [OK]

Hint: Trie stores prefixes explicitly, best for prefix search [OK]

Common Mistakes:

• Assuming hash map supports prefix search
• Ignoring performance cost of filtering large arrays
• Overestimating binary search for prefix retrieval