Group Anagrams Using Hash Map in DSA C++ - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare the hash map that groups anagrams.

DSA C++

unordered_map<string, vector<string>> [1];

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AhashMap

BgroupMap

CanagramGroups

DmapAnagrams

Attempts:

3 left

💡 Hint

Common Mistakes

Using generic names like 'hashMap' which are less descriptive.

Using names that do not relate to grouping anagrams.

The variable name 'anagramGroups' clearly represents the purpose of the hash map to group anagrams.

2fill in blank

medium

Complete the code to sort the string to use as a key for grouping anagrams.

DSA C++

string key = [1];

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Aword

BsortedWord

Csort(word)

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to assign the result of sort() directly to a string variable.

Using an undefined variable name.

We first assign the original word to 'key' before sorting it. The sorting happens in the next line.

3fill in blank

hard

Fix the error in sorting the string key.

DSA C++

sort([1].begin(), [1].end());

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AsortedWord

Bword

Canagram

Dkey

Attempts:

3 left

💡 Hint

Common Mistakes

Sorting the original word instead of the key.

Using an undefined variable.

We sort the 'key' string to get the sorted version used as the hash map key.

4fill in blank

hard

Fill both blanks to insert the original word into the correct group in the hash map.

DSA C++

anagramGroups[[1]].[2](word);

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Akey

Bpush_back

Cinsert

Dappend

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'insert' which is not a vector method for adding at the end.

Using the original word as the map key instead of the sorted key.

We use the sorted 'key' to access the vector and 'push_back' to add the original word.

5fill in blank

hard

Fill all three blanks to create the final result vector from the hash map.

DSA C++

vector<vector<string>> result;
for (auto& [1] : anagramGroups) {
    result.[2]([1].[3]);
}

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Aentry

Bpush_back

Csecond

Dfirst

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'first' instead of 'second' to access the vector.

Using incorrect variable names in the loop.

We iterate over each map entry named 'entry', then push_back the vector stored in entry.second into result.

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