Trie Search Operation in DSA C - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Trie Search for Existing Word

What is the output of the following C code that searches for the word "cat" in a Trie after inserting "cat" and "car"?

DSA C

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define ALPHABET_SIZE 26

typedef struct TrieNode {
    struct TrieNode* children[ALPHABET_SIZE];
    int isEndOfWord;
} TrieNode;

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

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

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

int main() {
    TrieNode* root = getNode();
    insert(root, "cat");
    insert(root, "car");
    int result = search(root, "cat");
    printf("%d\n", result);
    return 0;
}

A 0

B Compilation error

C Segmentation fault

D 1

💡 Hint

Check if the search function returns 1 when the word is found.

The word "cat" was inserted, so searching for "cat" returns 1 indicating it exists in the Trie.

❓ Predict Output

intermediate

Output of Trie Search for Non-Existing Word

What is the output of the following C code that searches for the word "cap" in a Trie after inserting "cat" and "car"?

DSA C

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define ALPHABET_SIZE 26

typedef struct TrieNode {
    struct TrieNode* children[ALPHABET_SIZE];
    int isEndOfWord;
} TrieNode;

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

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

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

int main() {
    TrieNode* root = getNode();
    insert(root, "cat");
    insert(root, "car");
    int result = search(root, "cap");
    printf("%d\n", result);
    return 0;
}

A 0

B 1

C Segmentation fault

D Compilation error

💡 Hint

Check if the search function returns 0 when the word is not found.

The word "cap" was not inserted, so searching returns 0 indicating it does not exist in the Trie.

🧠 Conceptual

advanced

Understanding Trie Search Behavior

In a Trie, what does the search operation return if the searched word is a prefix of an inserted word but not marked as a complete word itself?

A Returns -1 indicating partial match

B Causes a runtime error due to incomplete word

C Returns 0 because the prefix is not marked as a complete word

D Returns 1 because the prefix exists in the Trie

💡 Hint

Check the isEndOfWord flag in the Trie node after traversal.

Search returns 0 if the node exists but isEndOfWord is not set, meaning the prefix is not a complete word.

🔧 Debug

advanced

Identify the Bug in Trie Search Function

What error will occur when running this search function if the input word contains uppercase letters?

DSA C

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

A Index out of bounds leading to undefined behavior

B Compilation error due to invalid pointer usage

C No error, works correctly

D Segmentation fault due to null pointer dereference

💡 Hint

Consider how uppercase letters affect the index calculation.

Uppercase letters have ASCII values less than 'a', causing negative index and out of bounds access.

🚀 Application

expert

Resulting Trie Node Count After Insertions

After inserting the words "bat", "bath", "baton", and "bar" into an empty Trie, how many Trie nodes are created in total?

A 10

B 8

C 9

D 11

💡 Hint

Count nodes shared by prefixes only once.

The words share prefixes 'bat' and 'ba', nodes are reused. Total nodes created are 8.

Practice

1. What does the search operation in a Trie data structure primarily check for?

easy

A. The number of words stored in the Trie

B. If the Trie is empty

C. If a given word exists in the Trie

D. The length of the longest word in the Trie

Solution

1. Step 1: Understand the purpose of Trie search

   The search operation in a Trie is designed to check if a specific word is present by traversing nodes corresponding to each character.

2. Step 2: Identify what the search confirms

   If all characters of the word are found sequentially and the last node marks the end of a word, the word exists in the Trie.

3. Final Answer:

   If a given word exists in the Trie -> Option C

4. Quick Check:

   Trie search checks word existence = C [OK]

Hint: Search checks presence of full word in Trie [OK]

Common Mistakes:

• Confusing search with counting words
• Thinking search checks Trie emptiness
• Assuming search returns word length

2. Which of the following is the correct way to check if a character node exists in a Trie during search in C?

easy

A. if (node->children[c] == NULL)

B. if (node->children[c - 'a'] != NULL)

C. if (node->children[c + 'a'] != NULL)

D. if (node->children[c] != 0)

Solution

1. Step 1: Understand character indexing in Trie

   Characters are mapped to array indices by subtracting 'a' to get 0-based index for children array.

2. Step 2: Check for non-null child node

   To verify if a child node exists for character c, check if node->children[c - 'a'] is not NULL.

3. Final Answer:

   if (node->children[c - 'a'] != NULL) -> Option B

4. Quick Check:

   Index = c - 'a', check != NULL = B [OK]

Hint: Use c - 'a' to index children array in Trie [OK]

Common Mistakes:

• Using c directly as index without subtracting 'a'
• Checking for NULL with wrong comparison
• Adding 'a' to c instead of subtracting

3. Given the following Trie search code snippet in C, what will be the output if we search for "cat" after inserting "car" and "cat"?

bool search(struct TrieNode* root, const char* word) {
    struct TrieNode* node = root;
    for (int i = 0; word[i] != '\0'; i++) {
        int index = word[i] - 'a';
        if (node->children[index] == NULL) {
            return false;
        }
        node = node->children[index];
    }
    return node->isEndOfWord;
}

medium

A. true

B. false

C. Compilation error

D. Runtime error

Solution

1. Step 1: Understand inserted words and search target

   Words "car" and "cat" are inserted, so nodes for 'c', 'a', 'r' and 't' exist with isEndOfWord true at 'r' and 't'.

2. Step 2: Trace search for "cat"

   Search traverses 'c' -> 'a' -> 't' nodes, all present, and node->isEndOfWord is true at 't', so returns true.

3. Final Answer:

   true -> Option A

4. Quick Check:

   Inserted "cat" found = true [OK]

Hint: Search returns true if word was inserted before [OK]

Common Mistakes:

• Assuming search returns false if similar word exists
• Confusing isEndOfWord with node presence
• Ignoring null checks in loop

4. Identify the bug in the following Trie search function in C:

bool search(struct TrieNode* root, const char* word) {
    struct TrieNode* node = root;
    int i = 0;
    while (word[i] != '\0') {
        int index = word[i] - 'a';
        if (node->children[index] == NULL) {
            return false;
        }
        node = node->children[index];
        i++;
    }
    return false;
}

medium

A. Infinite loop due to missing increment

B. Does not check for NULL root

C. Incorrect indexing of children array

D. Returns false even if word exists

Solution

1. Step 1: Analyze return statement after loop

   The function returns false after loop ends, ignoring if current node marks end of word.

2. Step 2: Identify missing check for word completion

   It should return node->isEndOfWord to confirm full word presence, but returns false always.

3. Final Answer:

   Returns false even if word exists -> Option D

4. Quick Check:

   Missing isEndOfWord check causes false return [OK]

Hint: Return node->isEndOfWord after loop to confirm word [OK]

Common Mistakes:

• Forgetting to check isEndOfWord at end
• Assuming node presence means word exists
• Ignoring root NULL check (less critical here)

5. You have a Trie storing words: "bat", "bath", "baton", "bar", and "barn". Which of the following search queries will return false? "bathe", "bar", "bat", "barn"

hard

A. "bathe"

B. "bar"

C. "bat"

D. "barn"

Solution

1. Step 1: Check each query word against inserted words

   "bathe" is not inserted; "bar", "bat", and "barn" are inserted words in the Trie.

2. Step 2: Understand Trie search returns false for non-inserted words

   Since "bathe" is not inserted, search will fail at 'h' or after, returning false.

3. Final Answer:

   "bathe" -> Option A

4. Quick Check:

   Non-inserted word returns false = "bathe" [OK]

Hint: Search false if word not fully inserted in Trie [OK]

Common Mistakes:

• Assuming prefix words always return true
• Confusing prefix presence with full word presence
• Ignoring isEndOfWord flag in search