Group Anagrams Using Hash Map in DSA C++ - Build from Scratch

Group Anagrams Using Hash Map

📖 Scenario: You work in a word game company. You need to group words that are anagrams of each other to help players find word matches quickly.

🎯 Goal: Build a program that groups a list of words into sets of anagrams using a hash map.

📋 What You'll Learn

Create a vector of strings with given words

Create a hash map to group anagrams by sorted word keys

Use a loop to fill the hash map with grouped anagrams

Print the grouped anagrams as vectors of strings

💡 Why This Matters

🌍 Real World

Grouping anagrams helps in word games, search engines, and spell checkers to find related words quickly.

💼 Career

Understanding hash maps and string manipulation is essential for software engineers working on text processing and data grouping tasks.

1

Create the list of words

Create a std::vector<std::string> called words with these exact entries: "eat", "tea", "tan", "ate", "nat", "bat".

DSA C++

#include 
#include 
#include 

int main() {
    // Create vector words with given entries
    // Your code here
}

Hint

Use std::vector<std::string> and initialize it with the exact words inside curly braces.

2

Create the hash map to group anagrams

Add a std::unordered_map<std::string, std::vector<std::string>> called anagramGroups to store groups of anagrams by their sorted word keys.

DSA C++

#include 
#include 
#include 
#include 

int main() {
    std::vector words = {"eat", "tea", "tan", "ate", "nat", "bat"};
    // Create unordered_map anagramGroups
    // Your code here
}

Hint

Include <unordered_map> and declare anagramGroups as a map from std::string to std::vector<std::string>.

3

Group words by sorted keys

Use a for loop with variable word to iterate over words. Inside the loop, create a std::string called sortedWord as a copy of word. Sort sortedWord using std::sort. Then add word to anagramGroups[sortedWord].

DSA C++

#include 
#include 
#include 
#include 
#include 

int main() {
    std::vector words = {"eat", "tea", "tan", "ate", "nat", "bat"};
    std::unordered_map> anagramGroups;
    // Loop over words and group anagrams
    // Your code here
}

Hint

Copy each word to sortedWord, sort it, then use it as a key to add the original word to anagramGroups.

4

Print the grouped anagrams

Use a for loop with variables key and group to iterate over anagramGroups. Inside the loop, print the words in group separated by spaces, then print a newline.

DSA C++

#include 
#include 
#include 
#include 
#include 

int main() {
    std::vector words = {"eat", "tea", "tan", "ate", "nat", "bat"};
    std::unordered_map> anagramGroups;
    for (const std::string& word : words) {
        std::string sortedWord = word;
        std::sort(sortedWord.begin(), sortedWord.end());
        anagramGroups[sortedWord].push_back(word);
    }
    // Print each group of anagrams
    // Your code here
}

Hint

Use a range-based for loop with structured bindings to print each vector of anagrams separated by spaces, then a newline.

Practice

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

easy

A. Use sorted strings as keys to group words with the same characters

B. Use the length of the string as the key to group words

C. Use the first character of the string as the key

D. Use the last character of the string as the key

Solution

1. Step 1: Understand anagram grouping

   Anagrams have the same characters in different orders, so sorting the string gives a unique key for all anagrams.

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 together.

3. Final Answer:

   Use sorted strings as keys to group words with the same characters -> Option A

4. Quick Check:

   Sorted string key = group anagrams [OK]

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

Common Mistakes:

• Using string length as key misses different anagrams
• Using first or last character groups unrelated words
• Not sorting strings before using as keys

2. Which of the following C++ code snippets correctly declares a hash map to group anagrams where the key is a string and the value is a vector of strings?

easy

A. std::unordered_map anagramGroups;

B. std::map> anagramGroups;

C. std::unordered_map> anagramGroups;

D. std::unordered_map anagramGroups;

Solution

1. Step 1: Identify key and value types

   The key should be a string (sorted word), and the value should be a vector of strings (group of anagrams).

2. Step 2: Match correct declaration

   std::unordered_map> anagramGroups; uses unordered_map with key as string and value as vector<string>, which is correct for grouping anagrams.

3. Final Answer:

   std::unordered_map> anagramGroups; -> Option C

4. Quick Check:

   Key=string, Value=vector<string> [OK]

Hint: Use unordered_map for grouping [OK]

Common Mistakes:

• Using int as key instead of string
• Using vector<char> as key (not hashable)
• Using string as value instead of vector<string>

3. What is the output of the following C++ code snippet?

std::vector words = {"eat", "tea", "tan", "ate", "nat", "bat"};
std::unordered_map> groups;
for (auto &word : words) {
    std::string sortedWord = word;
    std::sort(sortedWord.begin(), sortedWord.end());
    groups[sortedWord].push_back(word);
}
for (auto &pair : groups) {
    std::cout << "Group: ";
    for (auto &w : pair.second) std::cout << w << " ";
    std::cout << std::endl;
}

medium

A. Group: eat tea
Group: tan bat
Group: ate nat

B. Group: eat tea ate
Group: tan nat
Group: bat

C. Group: ate tea
Group: bat tan
Group: eat nat

D. Group: eat tan bat
Group: tea nat ate

Solution

1. Step 1: Sort each word and group by sorted key

   "eat", "tea", "ate" all sort to "aet" so grouped together; "tan", "nat" sort to "ant"; "bat" sorts to "abt".

2. Step 2: Print groups

   Groups printed are: "eat tea ate", "tan nat", and "bat".

3. Final Answer:

   Group: eat tea ate Group: tan nat Group: bat -> Option B

4. Quick Check:

   Sorted keys group anagrams correctly [OK]

Hint: Sort words, group by sorted key, print groups [OK]

Common Mistakes:

• Not sorting words before grouping
• Mixing groups by original word order
• Printing keys instead of grouped words

4. Identify the error in the following code snippet that tries to group anagrams:

std::vector words = {"abc", "bca", "cab"};
std::unordered_map> groups;
for (auto word : words) {
    std::string key = word;
    std::sort(word.begin(), word.end());
    groups[key].push_back(word);
}

medium

A. Sorting is done on 'word' but 'key' is used as map key, causing wrong grouping

B. The vector 'words' is not initialized correctly

C. The map should use vector<char> as key instead of string

D. The loop variable should be a reference to avoid copying

Solution

1. Step 1: Analyze sorting and key usage

   The code sorts 'word' but uses 'key' (unsorted) as the map key, so grouping is incorrect.

2. Step 2: Correct usage

   Sort 'key' and use it as the map key to group anagrams properly.

3. Final Answer:

   Sorting is done on 'word' but 'key' is used as map key, causing wrong grouping -> Option A

4. Quick Check:

   Sort key before using in map [OK]

Hint: Sort the key string, not the original word [OK]

Common Mistakes:

• Sorting wrong variable before using as key
• Using unsorted string as map key
• Ignoring reference vs copy in loop

5. Given a list of strings, how would you modify the grouping anagrams approach to handle case-insensitive grouping (e.g., "Eat" and "tea" should be grouped together)?

hard

A. Sort strings without changing case

B. Use the original string as key without any changes

C. Group by string length only

D. Convert each string to lowercase before sorting and using as key

Solution

1. Step 1: Normalize case for consistent keys

   Convert each string to lowercase so that case differences do not affect grouping.

2. Step 2: Sort the lowercase string and use as key

   Sorting the lowercase string ensures all case variants of anagrams map to the same key.

3. Step 3: Group anagrams using normalized keys

   Use the lowercase sorted string as the hash map key to group anagrams case-insensitively.

4. Final Answer:

   Convert each string to lowercase before sorting and using as key -> Option D

5. Quick Check:

   Lowercase + sort = case-insensitive grouping [OK]

Hint: Lowercase strings before sorting to ignore case [OK]

Common Mistakes:

• Not normalizing case before grouping
• Using original case strings as keys
• Grouping by length or unsorted strings