Trie Node Design and Initialization in DSA C++ - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a pointer array for 26 lowercase English letters in the TrieNode class.

DSA C++

class TrieNode {
public:
    TrieNode* children[[1]];
    bool isEndOfWord;
    TrieNode() {
        isEndOfWord = false;
        for (int i = 0; i < [1]; i++) {
            children[i] = nullptr;
        }
    }
};

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

A26

B25

C52

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using 25 instead of 26, which misses one letter.

Using 52 which is for uppercase and lowercase combined, but here only lowercase is needed.

The English lowercase alphabet has 26 letters, so the children array size must be 26.

2fill in blank

medium

Complete the constructor to initialize the isEndOfWord flag to false.

DSA C++

TrieNode() {
    isEndOfWord = [1];
    for (int i = 0; i < 26; i++) {
        children[i] = nullptr;
    }
}

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

Afalse

Btrue

CNULL

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Setting isEndOfWord to true by mistake.

Using NULL or 0 which are not boolean values.

When a TrieNode is created, it should not mark the end of a word, so isEndOfWord is false.

3fill in blank

hard

Fix the error in the loop that initializes children pointers to nullptr.

DSA C++

for (int i = 0; i < [1]; i++) {
    children[i] = nullptr;
}

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

ANULL

B26

Cchildren.size()

D25

Attempts:

3 left

💡 Hint

Common Mistakes

Using 25 which misses the last child pointer.

Using children.size() which is not valid for raw arrays.

The loop must run 26 times to cover all children pointers for letters 'a' to 'z'.

4fill in blank

hard

Fill both blanks to declare a TrieNode pointer and initialize it to nullptr.

DSA C++

TrieNode* [1] = [2];

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

Aroot

Bnullptr

CNULL

Dnew TrieNode()

Attempts:

3 left

💡 Hint

Common Mistakes

Using NULL instead of nullptr (older style).

Initializing with new TrieNode() instead of nullptr.

We declare a pointer named root and initialize it to nullptr to indicate it points to nothing initially.

5fill in blank

hard

Fill all three blanks to create a new TrieNode and assign it to root, then mark it as not end of word.

DSA C++

root = [1];
root->isEndOfWord = [2];
for (int i = 0; i < [3]; i++) {
    root->children[i] = nullptr;
}

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

Anew TrieNode()

Bfalse

C26

Dtrue

Attempts:

3 left

💡 Hint

Common Mistakes

Setting isEndOfWord to true by mistake.

Using wrong number for children count.

Not creating a new TrieNode object.

We create a new TrieNode object, set isEndOfWord to false, and initialize all 26 children pointers to nullptr.

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