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

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Trie Node Initialization

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

DSA Java

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

public class Main {
    public static void main(String[] args) {
        TrieNode node = new TrieNode();
        System.out.println(node.isEndOfWord + ", " + node.children[0]);
    }
}

A true, TrieNode@<hashcode>

B false, null

C false, TrieNode@<hashcode>

D true, null

💡 Hint

Check the default values of boolean and object array elements in Java.

The boolean field isEndOfWord is initialized to false by default. The children array elements are object references and default to null. So printing node.isEndOfWord and node.children[0] outputs false, null.

❓ Predict Output

intermediate

Trie Node Children Array Size

What is the length of the children array in the following TrieNode class after initialization?

DSA Java

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

public class Main {
    public static void main(String[] args) {
        TrieNode node = new TrieNode();
        System.out.println(node.children.length);
    }
}

A 26

B 0

C null

D 1

💡 Hint

Look at the size used when creating the children array.

The children array is created with size 26 to represent all lowercase English letters. So its length is 26.

🔧 Debug

advanced

Identify the Error in Trie Node Initialization

What error will the following Java code produce when trying to compile or run?

DSA Java

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

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

A Syntax error: missing semicolon

B NullPointerException at runtime

C No error, runs fine

D ArrayIndexOutOfBoundsException at runtime

💡 Hint

Check the loop boundary condition for array indexing.

The loop runs from 0 to 26 inclusive, but the array size is 26, so valid indices are 0 to 25. Accessing children[26] causes ArrayIndexOutOfBoundsException at runtime.

🧠 Conceptual

advanced

Purpose of isEndOfWord in Trie Node

What is the main purpose of the boolean field isEndOfWord in a Trie node?

A To mark if the node represents the end of a valid word

B To count how many words pass through this node

C To store the character value of the node

D To indicate if the node has any children

💡 Hint

Think about how a Trie distinguishes complete words from prefixes.

The isEndOfWord flag marks that the path to this node forms a complete word stored in the Trie. It helps differentiate words from prefixes.

❓ Predict Output

expert

Output After Inserting a Word in Trie

Given the following simplified Trie insertion code, what is the output after inserting the word "cat" and printing the isEndOfWord flags of the nodes for 'c', 'a', and 't'?

DSA Java

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

public class Main {
    static TrieNode root = new TrieNode();

    public static void insert(String word) {
        TrieNode node = root;
        for (char ch : word.toCharArray()) {
            int index = ch - 'a';
            if (node.children[index] == null) {
                node.children[index] = new TrieNode();
            }
            node = node.children[index];
        }
        node.isEndOfWord = true;
    }

    public static void main(String[] args) {
        insert("cat");
        System.out.println(root.children['c' - 'a'].isEndOfWord + ", " +
                           root.children['c' - 'a'].children['a' - 'a'].isEndOfWord + ", " +
                           root.children['c' - 'a'].children['a' - 'a'].children['t' - 'a'].isEndOfWord);
    }
}

A false, true, true

B true, false, true

C false, false, true

D true, true, true

💡 Hint

Only the last node of the inserted word has isEndOfWord set to true.

When inserting "cat", only the node for 't' is marked as end of word. Nodes for 'c' and 'a' are intermediate and have isEndOfWord false.

Practice

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

easy

A. To act as a pointer to the next node in a linked list

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

C. To hold the entire word as a string

D. To store integer values only

Solution

1. Step 1: Understand the role of Trie nodes

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

2. Step 2: Compare options with Trie node purpose

   Only To store characters and links to child nodes representing prefixes correctly describes storing characters and child links for prefixes.

3. Final Answer:

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

4. Quick Check:

   Trie node = character + child links [OK]

Hint: Trie nodes hold characters and children, not full words [OK]

Common Mistakes:

• Thinking Trie nodes store full words
• Confusing Trie nodes with linked list nodes
• Assuming Trie nodes store only integers

2. Which of the following is the correct way to declare a Trie node class with an array of 26 children nodes in Java?

easy

A. class TrieNode { TrieNode[] children = new TrieNode[26]; boolean isEnd; }

B. class TrieNode { TrieNode children = new TrieNode[26]; boolean isEnd; }

C. class TrieNode { TrieNode[] children = new TrieNode(); boolean isEnd; }

D. class TrieNode { TrieNode[] children = new TrieNode[25]; boolean isEnd; }

Solution

1. Step 1: Check array declaration syntax

   In Java, to declare an array of TrieNode objects, use TrieNode[] children = new TrieNode[26];

2. Step 2: Validate each option

   class TrieNode { TrieNode[] children = new TrieNode[26]; boolean isEnd; } correctly declares an array of 26 TrieNode references. Others have syntax errors or wrong sizes.

3. Final Answer:

   class TrieNode { TrieNode[] children = new TrieNode[26]; boolean isEnd; } -> Option A

4. Quick Check:

   Array of 26 children = correct declaration [OK]

Hint: Use TrieNode[] children = new TrieNode[26]; for English alphabets [OK]

Common Mistakes:

• Using single TrieNode instead of array
• Wrong array size (25 instead of 26)
• Incorrect array initialization syntax

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

class TrieNode {
    TrieNode[] children = new TrieNode[26];
    boolean isEnd = false;
}

public class Main {
    public static void main(String[] args) {
        TrieNode node = new TrieNode();
        System.out.println(node.children[0]);
        System.out.println(node.isEnd);
    }
}

medium

A. null
false

B. TrieNode@hashcode
true

C. null
true

D. Compilation error

Solution

1. Step 1: Understand default initialization of array elements

   In Java, an array of objects initializes elements to null by default.

2. Step 2: Check boolean default value

   Boolean isEnd is explicitly set to false, so it prints false.

3. Final Answer:

   null false -> Option A

4. Quick Check:

   Array elements null, boolean false by default [OK]

Hint: Uninitialized object arrays print null, booleans print false [OK]

Common Mistakes:

• Expecting children[0] to be a TrieNode instance
• Assuming boolean defaults to true
• Thinking code causes compilation error

4. Identify the error in the following TrieNode class definition and suggest the fix:

class TrieNode {
    TrieNode children = new TrieNode[26];
    boolean isEnd;
}

medium

A. children array size should be 25 instead of 26

B. isEnd should be initialized to true

C. children should be an array: TrieNode[] children = new TrieNode[26];

D. No error, code is correct

Solution

1. Step 1: Analyze children declaration

   The code declares children as a single TrieNode but tries to assign an array, which is invalid.

2. Step 2: Correct the declaration

   children should be declared as an array: TrieNode[] children = new TrieNode[26];

3. Final Answer:

   children should be an array: TrieNode[] children = new TrieNode[26]; -> Option C

4. Quick Check:

   Array declaration mismatch causes error [OK]

Hint: Declare children as TrieNode[] for arrays, not single TrieNode [OK]

Common Mistakes:

• Assigning array to single object variable
• Ignoring array syntax errors
• Assuming boolean needs initialization to true

5. You want to design a TrieNode class that supports lowercase English letters and efficiently checks if a node marks the end of a word. Which of the following is the best design choice?

hard

A. Use a String to store the full word at each node and a list of children nodes

B. Use an integer count of words passing through and no children array

C. Use a HashMap for children and no isEnd flag

D. Use a boolean isEnd flag and an array of 26 TrieNode children initialized to null

Solution

1. Step 1: Consider efficient storage for fixed alphabet

   For lowercase English letters, an array of size 26 is memory efficient and fast for indexing.

2. Step 2: Importance of isEnd flag

   The isEnd boolean marks if the node completes a word, essential for word recognition.

3. Final Answer:

   Use a boolean isEnd flag and an array of 26 TrieNode children initialized to null -> Option D

4. Quick Check:

   Array + isEnd flag = best for fixed alphabets [OK]

Hint: Use array + boolean flag for fixed alphabets, HashMap for dynamic sets [OK]

Common Mistakes:

• Storing full words at each node wastes memory
• Omitting isEnd flag loses word boundary info
• Using HashMap unnecessarily for fixed small alphabets