Group Anagrams Using Hash Map in DSA Javascript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an anagram?

An anagram is a word or phrase made by rearranging the letters of another word or phrase, using all original letters exactly once.

beginner

Why do we use a hash map to group anagrams?

A hash map helps group words by a common key, such as sorted letters, so all anagrams share the same key and can be collected together easily.

beginner

How do you create a key for an anagram group in a hash map?

Sort the letters of the word alphabetically and use the sorted string as the key in the hash map.

intermediate

What is the time complexity of grouping anagrams using a hash map with sorted keys?

The time complexity is O(N * K log K), where N is the number of words and K is the maximum length of a word (due to sorting each word).

beginner

Explain the main steps to group anagrams using a hash map.

1. Initialize an empty hash map.
2. For each word, sort its letters to create a key.
3. Add the word to the list in the hash map under that key.
4. Return all grouped lists from the hash map.

What is the key used in the hash map to group anagrams?

A First letter of the word

B Length of the word

C Sorted letters of the word

D Number of vowels in the word

The sorted letters of the word form a unique key for all anagrams.

Which data structure is best to group anagrams efficiently?

A Hash map

B Stack

C Queue

D Linked list

Hash map allows quick grouping by keys, ideal for grouping anagrams.

What operation is performed on each word to create the hash map key?

A Reversing the word

B Sorting the letters

C Counting vowels

D Converting to uppercase

Sorting letters ensures all anagrams have the same key.

If the input list is ['bat', 'tab', 'eat', 'tea'], how many groups of anagrams will be formed?

A 2

B 1

C 3

D 4

There are two groups: ['bat', 'tab'] and ['eat', 'tea'].

What is the output format after grouping anagrams?

A Dictionary with word counts

B Single string of all words

C Sorted list of words

D List of lists, each containing anagrams

Output is a list of lists, each list contains words that are anagrams.

Describe how to group anagrams using a hash map step-by-step.

Think about how sorting helps identify anagrams.

Explain why sorting letters of a word is useful in grouping anagrams.

Consider what makes two words anagrams.

Practice

1. What is the main idea behind grouping anagrams using a hash map?

easy

A. Group words by their length using a hash map

B. Count the number of vowels in each word and group by count

C. Use the first letter of each word as the key in the hash map

D. Use sorted strings as keys to group words with the same letters

Solution

1. Step 1: Understand anagrams

   Anagrams are words with the same letters in different orders, so sorting letters gives a unique key.

2. Step 2: Use sorted string as hash map key

   By sorting each word and using it as a key, all anagrams map to the same key and can be grouped.

3. Final Answer:

   Use sorted strings as keys to group words with the same letters -> Option D

4. Quick Check:

   Sorted word keys group anagrams = C [OK]

Hint: Sort each word to find its anagram group key [OK]

Common Mistakes:

• Grouping by word length only
• Using first letter as key
• Counting vowels instead of letters

2. Which of the following JavaScript code snippets correctly initializes an empty hash map to group anagrams?

easy

A. const map = {};

B. const map = Map();

C. const map = [];

D. const map = new Set();

Solution

1. Step 1: Identify hash map initialization in JavaScript

   JavaScript objects ({}) are commonly used as hash maps for key-value pairs.

2. Step 2: Analyze the options

   {} creates a simple hash map using bracket notation like map[key]. Map() without new throws TypeError. [] creates an array unsuitable for key-value pairs. new Set() stores unique values, not mappings.

3. Final Answer:

   const map = {}; -> Option A

4. Quick Check:

   Object literal {} is simplest hash map = A [OK]

Hint: Use {} to create a simple hash map in JavaScript [OK]

Common Mistakes:

• Using array [] instead of object
• Using Set which doesn't store key-value pairs
• Confusing Map and object initialization

3. What is the output of the following code?

function groupAnagrams(words) {
  const map = {};
  for (const word of words) {
    const sorted = word.split('').sort().join('');
    if (!map[sorted]) map[sorted] = [];
    map[sorted].push(word);
  }
  return Object.values(map);
}

console.log(groupAnagrams(['bat', 'tab', 'eat', 'tea', 'ate']));

medium

A. SyntaxError

B. [['bat'], ['tab'], ['eat'], ['tea'], ['ate']]

C. [['bat', 'tab'], ['eat', 'tea', 'ate']]

D. [['bat', 'tab', 'eat', 'tea', 'ate']]

Solution

1. Step 1: Sort each word and group in map

   Words 'bat' and 'tab' both sort to 'abt', so grouped together. 'eat', 'tea', 'ate' all sort to 'aet', grouped together.

2. Step 2: Return grouped anagrams as array of arrays

   Object.values(map) returns arrays of grouped words: [['bat', 'tab'], ['eat', 'tea', 'ate']].

3. Final Answer:

   [['bat', 'tab'], ['eat', 'tea', 'ate']] -> Option C

4. Quick Check:

   Sorted keys group words = [['bat','tab'],['eat','tea','ate']] [OK]

Hint: Sort words, group by sorted key, return grouped arrays [OK]

Common Mistakes:

• Returning flat list instead of grouped arrays
• Not sorting words before grouping
• Syntax errors from missing brackets

4. Find the error in this code snippet for grouping anagrams:

function groupAnagrams(words) {
  const map = {};
  for (const word of words) {
    const sorted = word.sort().join('');
    if (!map[sorted]) map[sorted] = [];
    map[sorted].push(word);
  }
  return Object.values(map);
}

medium

A. map should be initialized as a Map object

B. word.sort() is invalid because word is a string, not an array

C. Missing return statement

D. Using Object.values on map is incorrect

Solution

1. Step 1: Identify the error in sorting

   word is a string, and strings do not have a sort() method. It must be split into an array first.

2. Step 2: Correct approach to sort letters

   Use word.split('').sort().join('') to sort letters of the string correctly.

3. Final Answer:

   word.sort() is invalid because word is a string, not an array -> Option B

4. Quick Check:

   Strings need split() before sort() = B [OK]

Hint: Split string to array before sorting letters [OK]

Common Mistakes:

• Calling sort() directly on string
• Not splitting string before sorting
• Confusing Map object with plain object

5. Given the input array ['listen', 'silent', 'enlist', 'google', 'gogole', 'abc', 'cab'], which of the following is the correct grouped anagrams output using a hash map approach?

hard

A. [['listen', 'silent', 'enlist'], ['google', 'gogole'], ['abc', 'cab']]

B. [['listen', 'silent'], ['enlist', 'google'], ['gogole', 'abc', 'cab']]

C. [['listen'], ['silent'], ['enlist'], ['google'], ['gogole'], ['abc'], ['cab']]

D. [['listen', 'silent', 'enlist', 'google', 'gogole', 'abc', 'cab']]

Solution

1. Step 1: Sort each word to find keys

   'listen', 'silent', 'enlist' all sort to 'eilnst'; 'google', 'gogole' sort to 'eggloo'; 'abc', 'cab' sort to 'abc'.

2. Step 2: Group words by sorted keys

   Group words with same sorted key together: [['listen', 'silent', 'enlist'], ['google', 'gogole'], ['abc', 'cab']].

3. Final Answer:

   [['listen', 'silent', 'enlist'], ['google', 'gogole'], ['abc', 'cab']] -> Option A

4. Quick Check:

   Sorted keys group anagrams correctly = A [OK]

Hint: Sort letters, group by key, collect arrays [OK]

Common Mistakes:

• Mixing groups with different sorted keys
• Grouping by word length instead of letters
• Returning flat list instead of grouped arrays