Word Search in Trie in DSA C# - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to insert a character into the Trie node's children dictionary.

DSA C#

node.Children[[1]] = new TrieNode();

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

Aword

Bword[i]

Ci

Dnode

Attempts:

3 left

💡 Hint

Common Mistakes

Using the whole word instead of a single character.

Using the index i instead of the character at that index.

We insert the character at index i of the word as the key in the children dictionary.

2fill in blank

medium

Complete the code to check if the current character exists in the Trie node's children.

DSA C#

if (!node.Children.ContainsKey([1])) return false;

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

Aword

Bi

Cword[i]

Dnode

Attempts:

3 left

💡 Hint

Common Mistakes

Checking for the whole word instead of a character.

Using the index i directly instead of the character at that index.

We check if the children dictionary contains the current character of the word.

3fill in blank

hard

Fix the error in the search method to move to the next Trie node correctly.

DSA C#

node = node.Children[[1]];

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

Aword[i]

Bword

Ci

Dnode

Attempts:

3 left

💡 Hint

Common Mistakes

Using the whole word as key instead of a character.

Using the index i instead of the character at that index.

We must use the current character of the word to get the next node from children.

4fill in blank

hard

Fill both blanks to complete the Trie insertion loop correctly.

DSA C#

for (int [1] = 0; [1] < word.[2]; [1]++) {
    // insertion logic
}

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Ai

BLength

Ccount

Dlen

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'count' or 'len' which are not valid string properties in C#.

Using a variable name other than 'i' for the loop index.

The loop variable is 'i' and the length property of string is 'Length' in C#.

5fill in blank

hard

Fill both blanks to complete the Trie search method's return statement correctly.

DSA C#

return node.[1] && index == word.[2];

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AIsWord

BLength

CCount

DIsEnd

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'Count' which is not a string property.

Using 'IsEnd' instead of 'IsWord' for the node property.

We check if the current node marks the end of a word with 'IsWord' and if index equals word.Length.

Practice

1. What is the main purpose of using a Trie data structure in word search problems?

easy

A. To efficiently store and search words by their prefixes

B. To sort words alphabetically

C. To store words in a linked list

D. To perform arithmetic operations on words

Solution

1. Step 1: Understand Trie structure

   A Trie stores characters of words in a tree-like structure where each node represents a character.

2. Step 2: Identify Trie usage in word search

   It allows quick prefix-based search, making it efficient to find words starting with given letters.

3. Final Answer:

   To efficiently store and search words by their prefixes -> Option A

4. Quick Check:

   Trie = prefix search [OK]

Hint: Trie is best for prefix-based word searches [OK]

Common Mistakes:

• Confusing Trie with sorting structures
• Thinking Trie stores words linearly
• Assuming Trie is for arithmetic

2. Which of the following is the correct way to declare a Trie node class in C#?

easy

A. public class TrieNode { public Dictionary Children = new Dictionary(); public bool IsWordEnd = false; }

B. public class TrieNode { public List<int> Children; public bool IsWordEnd; }

C. public class TrieNode { public string Children; public int IsWordEnd; }

D. public class TrieNode { public int Children; public bool IsWordEnd; }

Solution

1. Step 1: Check data types for children

   Trie children should map characters to TrieNode objects, so Dictionary is correct.

2. Step 2: Check word end marker

   Boolean flag IsWordEnd indicates if node ends a word, so bool is correct type.

3. Final Answer:

   public class TrieNode { public Dictionary Children = new Dictionary(); public bool IsWordEnd = false; } -> Option A

4. Quick Check:

   Children dictionary + bool flag = public class TrieNode { public Dictionary Children = new Dictionary(); public bool IsWordEnd = false; } [OK]

Hint: Trie node children map chars to nodes with bool end flag [OK]

Common Mistakes:

• Using List<int> instead of Dictionary
• Using string or int for children
• Using int instead of bool for IsWordEnd

3. Given the following C# code snippet for searching a word in a Trie, what will be the output?

Trie trie = new Trie();
trie.Insert("cat");
trie.Insert("car");
trie.Insert("dog");

bool result = trie.Search("cap");
Console.WriteLine(result);

medium

A. True

B. False

C. Compilation error

D. Runtime exception

Solution

1. Step 1: Understand inserted words

   Words "cat", "car", and "dog" are inserted into the Trie.

2. Step 2: Search for "cap"

   "cap" shares prefix "ca" but 'p' is not inserted after 'a', so search returns false.

3. Final Answer:

   False -> Option B

4. Quick Check:

   Word not found = False [OK]

Hint: Search returns false if full word path missing [OK]

Common Mistakes:

• Assuming prefix match means word found
• Expecting true for partial matches
• Confusing compilation/runtime errors

4. Identify the error in the following C# Trie search method:

public bool Search(string word) {
    TrieNode node = root;
    foreach (char c in word) {
        if (!node.Children.ContainsKey(c))
            return false;
        node = node.Children[c];
    }
    return true;
}

medium

A. It does not check for null root

B. It throws NullReferenceException if word is empty

C. It returns true even if the word is only a prefix, not a complete word

D. It uses wrong data type for node

Solution

1. Step 1: Analyze search logic

   The method checks if all characters exist in Trie nodes but does not check if last node marks end of a word.

2. Step 2: Identify missing condition

   Without checking node.IsWordEnd, it returns true for prefixes that are not complete words.

3. Final Answer:

   It returns true even if the word is only a prefix, not a complete word -> Option C

4. Quick Check:

   Missing IsWordEnd check = It returns true even if the word is only a prefix, not a complete word [OK]

Hint: Always check IsWordEnd after traversal [OK]

Common Mistakes:

• Ignoring IsWordEnd flag
• Assuming prefix means word found
• Not handling empty strings

5. You want to find all words in a Trie that start with the prefix "pre". Which approach correctly uses the Trie to achieve this?

hard

A. Use a hash set to store all words and filter those starting with "pre"

B. Search each word in the Trie individually to check if it starts with "pre"

C. Insert all words starting with "pre" into a new Trie and search there

D. Traverse the Trie to the node representing 'e' in "pre", then perform a DFS to collect all words below

Solution

1. Step 1: Locate prefix node in Trie

   Traverse characters 'p', 'r', 'e' to reach the node representing last prefix character.

2. Step 2: Collect words using DFS

   From this node, perform depth-first search to find all complete words that start with "pre".

3. Final Answer:

   Traverse the Trie to the node representing 'e' in "pre", then perform a DFS to collect all words below -> Option D

4. Quick Check:

   Prefix node + DFS = Traverse the Trie to the node representing 'e' in "pre", then perform a DFS to collect all words below [OK]

Hint: Find prefix node, then DFS for words [OK]

Common Mistakes:

• Checking all words individually
• Using unrelated data structures
• Rebuilding Trie unnecessarily