Longest Word in Dictionary Using Trie in DSA C - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a new Trie node.

DSA C

struct TrieNode* getNode() {
    struct TrieNode* pNode = (struct TrieNode*)malloc(sizeof(struct TrieNode));
    pNode->isEndOfWord = [1];
    for (int i = 0; i < 26; i++)
        pNode->children[i] = NULL;
    return pNode;
}

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

A1

B0

C-1

DNULL

Attempts:

3 left

💡 Hint

Common Mistakes

Setting isEndOfWord to 1 by mistake.

Using NULL instead of 0 for a boolean flag.

The isEndOfWord flag should be initialized to 0 (false) when creating a new Trie node.

2fill in blank

medium

Complete the code to insert a word into the Trie.

DSA C

void insert(struct TrieNode* root, const char* key) {
    struct TrieNode* pCrawl = root;
    for (int level = 0; key[level] != '\0'; level++) {
        int index = key[level] - 'a';
        if (!pCrawl->children[[1]])
            pCrawl->children[index] = getNode();
        pCrawl = pCrawl->children[index];
    }
    pCrawl->isEndOfWord = 1;
}

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

ApCrawl

Blevel

Ckey[level]

Dindex

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'level' instead of 'index' to access children.

Using the character directly as an index.

We check if the child at position 'index' is NULL before creating a new node.

3fill in blank

hard

Fix the error in the function that checks if a word exists in the Trie.

DSA C

int search(struct TrieNode* root, const char* key) {
    struct TrieNode* pCrawl = root;
    for (int level = 0; key[level] != '\0'; level++) {
        int index = key[level] - 'a';
        if (!pCrawl->children[[1]])
            return 0;
        pCrawl = pCrawl->children[index];
    }
    return (pCrawl != NULL && pCrawl->isEndOfWord);
}

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

Aindex

Bkey[level]

Clevel

DpCrawl

Attempts:

3 left

💡 Hint

Common Mistakes

Checking children at 'level' instead of 'index'.

Using the character directly as an index.

We must check the child at 'index' to see if the path exists in the Trie.

4fill in blank

hard

Fill both blanks to implement the recursive function that finds the longest word in the Trie where all prefixes are valid words.

DSA C

void longestWordUtil(struct TrieNode* root, char* currentWord, int level, char* longestWord) {
    if (root->isEndOfWord) {
        currentWord[level] = '\0';
        if (strlen(currentWord) > strlen(longestWord)) {
            strcpy(longestWord, [1]);
        }
    }
    for (int i = 0; i < 26; i++) {
        if (root->children[i] && root->children[i]->isEndOfWord) {
            currentWord[level] = 'a' + [2];
            longestWordUtil(root->children[i], currentWord, level + 1, longestWord);
        }
    }
}

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

AcurrentWord

BlongestWord

Ci

Dlevel

Attempts:

3 left

💡 Hint

Common Mistakes

Copying longestWord to currentWord instead of the other way.

Using 'level' instead of 'i' to calculate the character.

We copy the currentWord to longestWord when a longer valid word is found, and use 'i' to get the character for the next level.

5fill in blank

hard

Fill all three blanks to complete the function that finds the longest word in the dictionary using the Trie.

DSA C

char* longestWord(char** words, int wordsSize) {
    struct TrieNode* root = getNode();
    for (int i = 0; i < wordsSize; i++) {
        insert(root, [1]);
    }
    char currentWord[1000];
    char* longestWord = (char*)malloc(1000 * sizeof(char));
    longestWord[0] = '\0';
    longestWordUtil(root, currentWord, 0, [2]);
    return [3];
}

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

Awords[i]

BlongestWord

DcurrentWord

Attempts:

3 left

💡 Hint

Common Mistakes

Passing currentWord instead of longestWord to longestWordUtil.

Returning currentWord instead of longestWord.

Inserting wrong variable instead of words[i].

Insert each word from words[i], pass longestWord to the utility function, and return longestWord at the end.

Practice

1. What is the main purpose of using a Trie data structure when finding the longest word in a dictionary?

easy

A. To sort words alphabetically

B. To count the total number of words

C. To efficiently store and search words by their prefixes

D. To reverse the words in the dictionary

Solution

1. Step 1: Understand Trie usage

   A Trie stores words by breaking them into characters and linking nodes for prefixes.

2. Step 2: Relate to longest word search

   This structure allows quick prefix checks, which helps find the longest word built from other words.

3. Final Answer:

   To efficiently store and search words by their prefixes -> Option C

4. Quick Check:

   Trie = prefix search [OK]

Hint: Trie is best for prefix-based word searches [OK]

Common Mistakes:

• Thinking Trie sorts words alphabetically
• Confusing Trie with simple arrays or lists
• Assuming Trie counts words only

2. Which of the following is the correct way to declare a Trie node structure in C for lowercase English letters?

easy

A. typedef struct TrieNode { struct TrieNode* children[26]; int isEndOfWord; } TrieNode;

B. typedef struct TrieNode { struct TrieNode* children[52]; int isEndOfWord; } TrieNode;

C. typedef struct TrieNode { struct TrieNode* children[10]; int isEndOfWord; } TrieNode;

D. typedef struct TrieNode { struct TrieNode* children; int isEndOfWord; } TrieNode;

Solution

1. Step 1: Identify children array size

   Lowercase English letters are 26, so children array size must be 26.

2. Step 2: Check structure fields

   Each node has an array of pointers and a flag for word end, matching typedef struct TrieNode { struct TrieNode* children[26]; int isEndOfWord; } TrieNode;.

3. Final Answer:

   typedef struct TrieNode { struct TrieNode* children[26]; int isEndOfWord; } TrieNode; -> Option A

4. Quick Check:

   26 children for a-z [OK]

Hint: Use 26 children for lowercase letters [OK]

Common Mistakes:

• Using 52 for uppercase + lowercase without need
• Using 10 children for digits only
• Not using array for children pointers

3. Given the following code snippet inserting words into a Trie and searching for the longest word built one character at a time, what will be the output?

char* words[] = {"a", "ap", "app", "appl", "apple", "apply"};
// Assume insert and search functions are correct
// Function finds longest word where all prefixes exist
printf("%s", longestWord(words, 6));

medium

A. apple

B. apply

C. appl

D. app

Solution

1. Step 1: Check all prefixes for each word

   Words like "apple" and "apply" have all prefixes present: a, ap, app, appl.

2. Step 2: Compare lengths of valid words

   "apple" and "apply" both have length 5, but "apple" comes first lexicographically.

3. Final Answer:

   apple -> Option A

4. Quick Check:

   Longest valid word = apple [OK]

Hint: Check all prefixes exist, pick longest lex order first [OK]

Common Mistakes:

• Choosing 'apply' ignoring lex order
• Picking shorter words like 'app'
• Not verifying all prefixes exist

4. The following code tries to find the longest word in a dictionary using a Trie but always returns an empty string. What is the bug?

void insert(TrieNode* root, char* word) {
    TrieNode* node = root;
    for (int i = 0; word[i]; i++) {
        int index = word[i] - 'a';
        if (!node->children[index])
            node->children[index] = createNode();
        node = node->children[index];
    }
    node->isEndOfWord = 1;
}

char* longestWord(char** words, int n) {
    TrieNode* root = createNode();
    for (int i = 0; i < n; i++)
        insert(root, words[i]);
    // Missing code to find longest word
    return "";
}

medium

A. The children array size is incorrect

B. The longestWord function does not search the Trie for the longest word

C. The insert function does not mark end of word correctly

D. The root node is not initialized

Solution

1. Step 1: Analyze insert function

   Insert correctly adds words and marks end of word, so no issue here.

2. Step 2: Check longestWord function

   It inserts words but returns empty string without searching Trie for longest word.

3. Final Answer:

   The longestWord function does not search the Trie for the longest word -> Option B

4. Quick Check:

   Missing search logic causes empty output [OK]

Hint: Check if search logic exists after insertion [OK]

Common Mistakes:

• Assuming insert alone finds longest word
• Ignoring missing traversal in longestWord
• Thinking root node is uninitialized

5. You want to find the longest word in a dictionary such that every prefix of the word is also a word in the dictionary. Which approach using a Trie is best to solve this efficiently?

hard

A. Use BFS on Trie without checking end of word flags

B. Sort words by length, then check each word's prefixes in a hash set

C. Insert words into Trie and count total nodes to find longest word

D. Insert all words into Trie, then do DFS checking only nodes marking end of words to build longest word

Solution

1. Step 1: Insert all words into Trie

   Build Trie with all dictionary words, marking end of word nodes.

2. Step 2: Use DFS to find longest word

   Traverse Trie depth-first, only continuing on nodes that mark end of words to ensure all prefixes exist.

3. Step 3: Track longest word during traversal

   Keep updating longest word found by length and lex order during DFS.

4. Final Answer:

   Insert all words into Trie, then do DFS checking only nodes marking end of words to build longest word -> Option D

5. Quick Check:

   Trie + DFS + endOfWord check = longest word [OK]

Hint: Use DFS on Trie nodes with endOfWord true only [OK]

Common Mistakes:

• Ignoring prefix checks during traversal
• Using BFS without endOfWord validation
• Relying on sorting without Trie