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

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a child nodes array for a Trie node.

DSA C#

public TrieNode() {
    children = new TrieNode[[1]];
}

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

A26

B52

C10

D128

Attempts:

3 left

💡 Hint

Common Mistakes

Using 52 for uppercase and lowercase combined when only lowercase is needed.

Using 10 which is for digits, not letters.

A Trie node for lowercase English letters uses an array of size 26 for children nodes.

2fill in blank

medium

Complete the code to initialize the end-of-word flag in the Trie node constructor.

DSA C#

public TrieNode() {
    children = new TrieNode[26];
    [1] = false;
}

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

AwordEnd

BisEndOfWord

CendFlag

DisWord

Attempts:

3 left

💡 Hint

Common Mistakes

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

Using names that do not clearly indicate the purpose.

The common name for the flag marking the end of a word is 'isEndOfWord'.

3fill in blank

hard

Fix the error in the TrieNode constructor by completing the initialization of all children to null.

DSA C#

public TrieNode() {
    children = new TrieNode[26];
    isEndOfWord = false;
    for (int i = 0; i < 26; i++) {
        children[[1]] = null;
    }
}

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

Ai

Bchildren

CisEndOfWord

Dnull

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong variable name inside the array index.

Trying to assign null to the whole array instead of each element.

The loop index variable 'i' is used to access each child node to set it to null.

4fill in blank

hard

Fill both blanks to complete the method that checks if a Trie node has any children.

DSA C#

public bool HasChildren() {
    for (int [1] = 0; [1] < 26; [1]++) {
        if (children[[2]] != null) {
            return true;
        }
    }
    return false;
}

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

Ai

Bj

Ck

Dindex

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variables for loop and array index causing errors.

Using variables not declared in the loop.

The same loop variable 'i' is used for iteration and indexing children array.

5fill in blank

hard

Fill all three blanks to complete the Insert method for adding a word to the Trie.

DSA C#

public void Insert(string word) {
    TrieNode node = this;
    foreach (char [1] in word) {
        int index = [2] - 'a';
        if (node.children[index] == null) {
            node.children[index] = new TrieNode();
        }
        node = node.children[[3]];
    }
    node.isEndOfWord = true;
}

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

Ac

Bch

Cindex

Dchar

Attempts:

3 left

💡 Hint

Common Mistakes

Using the character variable directly in the array index instead of 'index'.

Using reserved words or types as variable names.

Use the character variable 'c' in the foreach loop and index calculation, and 'index' for array access.

Practice

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

easy

A. To act as a queue for BFS traversal

B. To store integer values for sorting

C. To hold a single string value only

D. To store characters and links to child nodes representing prefixes

Solution

1. Step 1: Understand Trie node role

   A Trie node stores a character and links to child nodes representing possible next characters in words.

2. Step 2: Compare options

   Only To store characters and links to child nodes representing prefixes correctly describes this role; others describe unrelated data structures or incorrect uses.

3. Final Answer:

   To store characters and links to child nodes representing prefixes -> Option D

4. Quick Check:

   Trie node = store chars + children links [OK]

Hint: Trie nodes hold chars and children links [OK]

Common Mistakes:

• Thinking Trie nodes store full words only
• Confusing Trie nodes with array or list nodes
• Assuming Trie nodes hold integer values

2. Which of the following is the correct way to declare a Trie node class with a children array for 26 lowercase letters in C#?

easy

A. public class TrieNode { public TrieNode[] children = new TrieNode[26]; public bool isEndOfWord; }

B. public class TrieNode { public List<char> children = new List<char>(); public bool isEndOfWord; }

C. public class TrieNode { public Dictionary children = new Dictionary(); public bool isEndOfWord; }

D. public class TrieNode { public TrieNode children = null; public bool isEndOfWord; }

Solution

1. Step 1: Identify correct children storage

   For 26 lowercase letters, an array of size 26 is standard to store child nodes indexed by character.

2. Step 2: Check syntax correctness

   public class TrieNode { public TrieNode[] children = new TrieNode[26]; public bool isEndOfWord; } declares a TrieNode array of size 26 and a boolean flag, which is correct syntax and design.

3. Final Answer:

   public class TrieNode { public TrieNode[] children = new TrieNode[26]; public bool isEndOfWord; } -> Option A

4. Quick Check:

   Children array size 26 = public class TrieNode { public TrieNode[] children = new TrieNode[26]; public bool isEndOfWord; } [OK]

Hint: Use TrieNode[] children = new TrieNode[26] for letters [OK]

Common Mistakes:

• Using List<char> instead of TrieNode array
• Using Dictionary unnecessarily
• Declaring children as single TrieNode instead of array

3. What will be the output of the following C# code snippet?

public class TrieNode {
    public TrieNode[] children = new TrieNode[26];
    public bool isEndOfWord = false;
}

public class Program {
    public static void Main() {
        TrieNode node = new TrieNode();
        Console.WriteLine(node.children[0] == null);
        Console.WriteLine(node.isEndOfWord);
    }
}

medium

A. false
False

B. true
True

C. true
False

D. false
True

Solution

1. Step 1: Check default initialization of children array

   In C#, arrays of reference types initialize elements to null by default, so children[0] is null, making the first print true.

2. Step 2: Check default value of isEndOfWord

   isEndOfWord is explicitly set to false, so printing it outputs false.

3. Final Answer:

   true False -> Option C

4. Quick Check:

   children[0] null = true, isEndOfWord false = false [OK]

Hint: Reference arrays default to null, bool defaults to false [OK]

Common Mistakes:

• Assuming children array elements are initialized to new nodes
• Confusing default bool value as true
• Misreading array initialization behavior

4. Identify the error in the following TrieNode class code and select the fix:

public class TrieNode {
    public TrieNode[] children;
    public bool isEndOfWord;

    public TrieNode() {
        children = new TrieNode[25];
        isEndOfWord = false;
    }
}

medium

A. Remove constructor and initialize fields inline

B. Change array size from 25 to 26 to cover all lowercase letters

C. Declare children as List<TrieNode> instead of array

D. Initialize isEndOfWord to true in constructor

Solution

1. Step 1: Check children array size

   For 26 lowercase English letters, children array must be size 26, not 25.

2. Step 2: Evaluate other options

   isEndOfWord should start false; List is optional but not required; constructor is fine.

3. Final Answer:

   Change array size from 25 to 26 to cover all lowercase letters -> Option B

4. Quick Check:

   Children array size must be 26 [OK]

Hint: Children array size must match alphabet count (26) [OK]

Common Mistakes:

• Using wrong array size causing index errors
• Setting isEndOfWord true by default
• Unnecessarily changing data structure type

5. You want to design a TrieNode class that supports both lowercase and uppercase English letters (52 total). Which is the best way to initialize the children array in C#?

hard

A. public Dictionary children = new Dictionary(); // dynamic keys for all letters

B. public TrieNode[] children = new TrieNode[26]; // only lowercase letters supported

C. public TrieNode[] children = new TrieNode[52]; // index 0-25 for 'a'-'z', 26-51 for 'A'-'Z'

D. public List<TrieNode> children = new List<TrieNode>(52); // list with capacity 52

Solution

1. Step 1: Consider fixed array size for 52 letters

   Using a fixed array of size 52 is possible but requires careful indexing and is error-prone.

2. Step 2: Evaluate dynamic dictionary approach

   A Dictionary with char keys allows flexible, clear mapping for both uppercase and lowercase letters without manual indexing.

3. Final Answer:

   public Dictionary children = new Dictionary(); // dynamic keys for all letters -> Option A

4. Quick Check:

   Dictionary best for mixed case letters [OK]

Hint: Use Dictionary for mixed letter sets [OK]

Common Mistakes:

• Using fixed array without clear indexing for uppercase
• Using List without key mapping
• Ignoring uppercase letters support