Trie Node Design and Initialization in DSA C++ - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Trie Node Initialization

What is the output of the following C++ code that initializes a Trie node and prints its initial state?

DSA C++

struct TrieNode {
    bool isEndOfWord;
    TrieNode* children[26];
    TrieNode() {
        isEndOfWord = false;
        for (int i = 0; i < 26; i++) {
            children[i] = nullptr;
        }
    }
};

#include <iostream>
int main() {
    TrieNode node;
    std::cout << node.isEndOfWord << " ";
    int count = 0;
    for (int i = 0; i < 26; i++) {
        if (node.children[i] == nullptr) count++;
    }
    std::cout << count << std::endl;
    return 0;
}

A 0 26

B 1 0

C 0 0

D 1 26

💡 Hint

Check the default values assigned in the constructor for isEndOfWord and children array.

The constructor sets isEndOfWord to false (which prints as 0) and initializes all 26 children pointers to nullptr. Counting the null pointers gives 26.

🧠 Conceptual

intermediate

Purpose of Children Array in Trie Node

What is the main purpose of the children array in a Trie node?

A To keep track of the parent node of the current node

B To store the frequency of words ending at this node

C To store pointers to child nodes representing each possible character

D To store the actual characters of the word at this node

💡 Hint

Think about how a Trie branches for each letter.

The children array holds pointers to child nodes for each character (usually 26 for lowercase English letters). This allows branching to next letters in stored words.

🔧 Debug

advanced

Identify the Bug in Trie Node Initialization

What error will occur when running this Trie node constructor code?

DSA C++

struct TrieNode {
    bool isEndOfWord;
    TrieNode* children[26];
    TrieNode() {
        isEndOfWord = false;
        for (int i = 0; i <= 26; i++) {
            children[i] = nullptr;
        }
    }
};

A Logical error: isEndOfWord not initialized

B No error, code runs correctly

C Compilation error due to missing semicolon

D Runtime error due to out-of-bounds array access

💡 Hint

Check the loop boundary condition for the children array initialization.

The loop uses i <= 26 which accesses index 26, but valid indices are 0 to 25. This causes out-of-bounds access leading to runtime error.

📝 Syntax

advanced

Correct Syntax for Trie Node Constructor Initialization

Which option correctly initializes all children pointers to nullptr in the TrieNode constructor?

DSA C++

struct TrieNode {
    bool isEndOfWord;
    TrieNode* children[26];
    TrieNode() {
        isEndOfWord = false;
        // Initialize children
    }
};

A for (int i = 0; i < 26; i++) children[i] = nullptr;

B for int i = 0; i < 26; i++ children[i] = nullptr;

C for (int i = 0; i <= 26; i++) children[i] = nullptr;

D for (int i = 1; i < 26; i++) children[i] = nullptr;

💡 Hint

Check for correct for-loop syntax and valid array indices.

Option A uses correct syntax and valid indices 0 to 25. Option A misses parentheses. Option A accesses index 26 (invalid). Option A skips index 0.

🚀 Application

expert

Number of Nodes After Inserting Words in Trie

Given the following insertions into an empty Trie, how many Trie nodes exist after inserting the words "cat", "car", and "dog"?

DSA C++

struct TrieNode {
    bool isEndOfWord;
    TrieNode* children[26];
    TrieNode() {
        isEndOfWord = false;
        for (int i = 0; i < 26; i++) children[i] = nullptr;
    }
};

// Insert words: "cat", "car", "dog"

A 6

B 8

C 7

D 9

💡 Hint

Count nodes shared by common prefixes only once.

"cat" and "car" share 'c' and 'a' nodes, so those nodes are shared. The nodes are:
Root (1)
c (2)
a (3)
t (4) for "cat"
r (5) for "car"
d (6) for "dog"
o (7) for "dog"
g (8) for "dog"
Total nodes = 8.

Practice

1. What is the primary purpose of a Trie node in a Trie data structure?

easy

A. To store children nodes representing characters of words

B. To store integer values for sorting

C. To hold pointers to parent nodes only

D. To keep track of the total number of words in the Trie

Solution

1. Step 1: Understand Trie node role

   A Trie node represents a character and links to children nodes for next characters.

2. Step 2: Identify node contents

   It stores children nodes to form paths representing words.

3. Final Answer:

   To store children nodes representing characters of words -> Option A

4. Quick Check:

   Trie node = children nodes [OK]

Hint: Trie nodes link characters as children nodes [OK]

Common Mistakes:

• Thinking nodes store whole words
• Confusing parent pointers as main content
• Assuming nodes store counts only

2. Which of the following is the correct way to initialize a Trie node's children array for lowercase English letters in C++?

easy

A. TrieNode* children = new TrieNode[26];

B. TrieNode children[26] = {nullptr};

C. vector children(26, nullptr);

D. vector<TrieNode> children(26);

Solution

1. Step 1: Understand children storage

   Children are pointers to nodes, so vector of TrieNode* is needed.

2. Step 2: Check initialization

   Initializing vector with 26 nullptr sets all children to null initially.

3. Final Answer:

   vector<TrieNode*> children(26, nullptr); -> Option C

4. Quick Check:

   Pointer vector with nullptr init = A [OK]

Hint: Use vector of pointers initialized to nullptr [OK]

Common Mistakes:

• Using array of objects instead of pointers
• Not initializing pointers to nullptr
• Confusing pointer arrays with object arrays

3. Consider this TrieNode struct in C++: <pre>struct TrieNode { vector<TrieNode*> children; bool isEndOfWord; TrieNode() : children(26, nullptr), isEndOfWord(false) {} }; TrieNode* root = new TrieNode(); root->children[0] = new TrieNode(); root->children[0]->isEndOfWord = true; What is the value of root->children[0]->isEndOfWord after this code runs?

medium

A. false

B. true

C. nullptr

D. Compilation error

Solution

1. Step 1: Trace node creation

   root is created with children initialized to nullptr and isEndOfWord false.

2. Step 2: Assign new node and set flag

   root->children[0] points to new TrieNode, then isEndOfWord is set true on that node.

3. Final Answer:

   true -> Option B

4. Quick Check:

   Set isEndOfWord true explicitly = A [OK]

Hint: Check explicit flag assignments after node creation [OK]

Common Mistakes:

• Assuming default false remains unchanged
• Confusing pointer with boolean value
• Thinking children array holds values, not pointers

4. Identify the error in this TrieNode constructor code snippet:

struct TrieNode {
  vector<TrieNode*> children;
  bool isEndOfWord;
  TrieNode() {
    children = vector<TrieNode*>(26);
    isEndOfWord = false;
  }
};

medium

A. Missing semicolon after struct

B. isEndOfWord is not initialized

C. Syntax error in vector declaration

D. Children vector elements are uninitialized pointers

Solution

1. Step 1: Analyze children vector initialization

   Vector is created with size 26 but elements are default-initialized pointers (not nullptr).

2. Step 2: Understand pointer initialization

   Pointers remain uninitialized (garbage), which can cause runtime errors.

3. Final Answer:

   Children vector elements are uninitialized pointers -> Option D

4. Quick Check:

   Uninitialized pointers in vector = D [OK]

Hint: Always initialize pointer vectors with nullptr [OK]

Common Mistakes:

• Assuming vector(26) sets nullptr automatically
• Ignoring pointer initialization risks
• Confusing syntax errors with logic errors

5. You want to design a TrieNode class in C++ that supports Unicode characters instead of just lowercase English letters. Which change is the best to handle children nodes efficiently?

hard

A. Use an unordered_map children instead of a fixed-size array

B. Increase the fixed array size to 65536 for all Unicode characters

C. Use a vector<TrieNode*> of size 128 and ignore other characters

D. Store children as a linked list of TrieNode pointers

Solution

1. Step 1: Understand Unicode character range

   Unicode has many characters, too large for fixed arrays.

2. Step 2: Choose efficient data structure

   unordered_map allows dynamic keys for any Unicode char efficiently.

3. Final Answer:

   Use an unordered_map<char32_t, TrieNode*> children instead of a fixed-size array -> Option A

4. Quick Check:

   Dynamic map for Unicode children = B [OK]

Hint: Use maps for large or sparse character sets [OK]

Common Mistakes:

• Using huge fixed arrays wastes memory
• Ignoring characters outside ASCII range
• Using linked lists causes slow lookups