Trie Node Design and Initialization in DSA C# - Execution Trace

Concept Flow - Trie Node Design and Initialization

Create new TrieNode

↓

Initialize children array to null

↓

Set isEndOfWord to false

↓

Node ready for insertion/search

This flow shows how a Trie node is created and initialized with empty children and a flag to mark word end.

Execution Sample

DSA C#

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

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

This code defines a Trie node with 26 children pointers and a boolean flag, initializing all children to null.

Execution Table

Step	Operation	Children Array State	isEndOfWord	Visual State
1	Create new TrieNode object	[null, null, ..., null] (26 times)	false	Node created with empty children and isEndOfWord false
2	Initialize children array elements to null	[null, null, ..., null] (26 times)	false	All children pointers set to null explicitly
3	Set isEndOfWord to false	[null, null, ..., null] (26 times)	false	Node ready for use in Trie operations
4	End of initialization	[null, null, ..., null] (26 times)	false	Initialization complete, node ready

💡 All children pointers are null and isEndOfWord is false, node fully initialized

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
children	undefined	[null x 26]	[null x 26]	[null x 26]	[null x 26]
isEndOfWord	undefined	false	false	false	false

Key Moments - 3 Insights

Why do we initialize all children to null explicitly?

Because the children array holds references to other TrieNodes, initializing to null means no child exists yet. See execution_table step 2 where children are set to null.

What does isEndOfWord represent and why start as false?

isEndOfWord marks if a node completes a valid word. It starts false because a new node doesn't represent a complete word yet. See execution_table step 3.

Why do we use an array of size 26 for children?

Because we assume only lowercase English letters a-z, each index corresponds to a letter. This fixed size simplifies indexing and lookup.

Visual Quiz - 3 Questions

Look at the execution_table, what is the state of children array after step 2?

A All elements are new TrieNodes

B All elements are null

C Array is empty

D Array contains random values

💡 Hint

Refer to execution_table row with Step 2 showing children array state

At which step is isEndOfWord set to false explicitly?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Check execution_table rows for isEndOfWord value changes

If we added uppercase letters, how would the children array size change?

A Increase to 52

B Decrease to 13

C Stay 26

D Become dynamic size

💡 Hint

Think about how many letters uppercase and lowercase alphabets have combined

Concept Snapshot

TrieNode Design:
- children: array of 26 TrieNode references (a-z)
- isEndOfWord: bool flag for word end
- Initialize children to null
- Set isEndOfWord false
- Ready for insert/search operations

Full Transcript

A Trie node is created by allocating an array of 26 children pointers, each initially null, representing letters a to z. The isEndOfWord boolean is set to false to mark that this node does not yet represent the end of a word. This initialization prepares the node for insertion and search operations in the Trie data structure.

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