Prefix Search Using Trie in DSA Java - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to insert a character into the children array of a Trie node.

DSA Java

node.children[[1] - 'a'] = new TrieNode();

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

Aindex

Bc

Cch

Di

Attempts:

3 left

💡 Hint

Common Mistakes

Using loop variable 'i' instead of character 'ch'.

Using an undefined variable 'index'.

The variable 'ch' represents the current character to insert, so subtracting 'a' gives the correct index.

2fill in blank

medium

Complete the code to check if a prefix exists in the Trie.

DSA Java

if (node.children[[1] - 'a'] == null) return false;

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

Ach

Bindex

Cc

Di

Attempts:

3 left

💡 Hint

Common Mistakes

Using loop index 'i' instead of character 'ch'.

Using an undefined variable 'index'.

We check the child node corresponding to the character 'ch' to see if the prefix path exists.

3fill in blank

hard

Fix the error in the method to return true if the prefix is found.

DSA Java

return node.[1];

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AisWord

BisPrefix

CisEnd

DendOfWord

Attempts:

3 left

💡 Hint

Common Mistakes

Returning 'isWord' which indicates a complete word, not prefix.

Using undefined flags like 'isEnd' or 'endOfWord'.

The method should return true if the prefix exists, so it returns the 'isWord' flag.

4fill in blank

hard

Fill both blanks to complete the loop that traverses the Trie for each character in the prefix.

DSA Java

for (char [1] : prefix.toCharArray()) {
    if (node.children[[2] - 'a'] == null) return false;
    node = node.children[[2] - 'a'];
}

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

Ach

Bc

Ci

Dindex

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variables for loop and indexing.

Using loop index 'i' which is not defined in this loop.

The loop variable 'ch' is used to access children nodes by subtracting 'a' to get the index.

5fill in blank

hard

Fill all three blanks to complete the method that inserts a word into the Trie.

DSA Java

for (int [1] = 0; [2] < word.length(); [3]++) {
    char ch = word.charAt(i);
    if (node.children[ch - 'a'] == null) {
        node.children[ch - 'a'] = new TrieNode();
    }
    node = node.children[ch - 'a'];
}
node.isWord = true;

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

Ai

Bj

Ck

Dindex

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names in loop parts causing errors.

Using undefined variables like 'index' or 'j' inconsistently.

The loop variable 'i' is used consistently for initialization, condition, and increment.

Practice

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

easy

A. To efficiently find all words that start with a given prefix

B. To sort words alphabetically

C. To store numbers in sorted order

D. To perform arithmetic operations quickly

Solution

1. Step 1: Understand what prefix search means

   Prefix search means finding all words that start with a certain beginning sequence of letters.

2. Step 2: Understand Trie's role in prefix search

   A Trie stores words in a tree where each node represents a letter, making it easy to find all words sharing the same prefix by traversing the tree.

3. Final Answer:

   To efficiently find all words that start with a given prefix -> Option A

4. Quick Check:

   Prefix search = efficient word lookup [OK]

Hint: Trie nodes represent letters for fast prefix lookup [OK]

Common Mistakes:

• Confusing Trie with sorting data structures
• Thinking Trie is for arithmetic operations
• Assuming Trie stores numbers only

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

easy

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

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

C. class TrieNode { String[] children = new String[26]; boolean isEndOfWord; }

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

Solution

1. Step 1: Identify the correct type for children nodes

   Each child should be a reference to another TrieNode, so the array type must be TrieNode[].

2. Step 2: Check the other parts of the class

   Boolean isEndOfWord marks if the node ends a word, which is correct in class TrieNode { TrieNode[] children = new TrieNode[26]; boolean isEndOfWord; }.

3. Final Answer:

   class TrieNode { TrieNode[] children = new TrieNode[26]; boolean isEndOfWord; } -> Option D

4. Quick Check:

   Children are TrieNode references [OK]

Hint: Children array holds TrieNode references, not primitives [OK]

Common Mistakes:

• Using int or char arrays instead of TrieNode array
• Using String array for children
• Forgetting to mark end of word

3. Given the following code snippet, what will be the output?

Trie trie = new Trie();
trie.insert("cat");
trie.insert("car");
trie.insert("dog");
System.out.println(trie.startsWith("ca"));
System.out.println(trie.startsWith("do"));
System.out.println(trie.startsWith("bat"));

medium

A. true
true
false

B. true
false
false

C. false
true
false

D. false
false
true

Solution

1. Step 1: Insert words into the Trie

   Words "cat", "car", and "dog" are inserted, so prefixes "ca" and "do" exist in the Trie.

2. Step 2: Check startsWith results

   startsWith("ca") returns true because "cat" and "car" start with "ca".
   startsWith("do") returns true because "dog" starts with "do".
   startsWith("bat") returns false because no word starts with "bat".

3. Final Answer:

   true true false -> Option A

4. Quick Check:

   Prefix "ca" and "do" found, "bat" not found [OK]

Hint: Check inserted words' prefixes carefully [OK]

Common Mistakes:

• Assuming startsWith returns false if word not complete
• Confusing startsWith with search for whole word
• Mixing up true/false outputs

4. The following Java method is intended to check if a prefix exists in a Trie, but it has a bug. What is the bug?

public boolean startsWith(String prefix) {
    TrieNode node = root;
    for (int i = 0; i <= prefix.length(); i++) {
        char c = prefix.charAt(i);
        if (node.children[c - 'a'] == null) return false;
        node = node.children[c - 'a'];
    }
    return true;
}

medium

A. The root node is not initialized before the loop

B. The loop condition should be < instead of <= to avoid IndexOutOfBoundsException

C. The method should return false at the end instead of true

D. The character indexing is wrong; should use c + 'a'

Solution

1. Step 1: Analyze the for-loop condition

   The loop runs from i = 0 to i <= prefix.length(), which means it tries to access prefix.charAt(prefix.length()), causing an IndexOutOfBoundsException.

2. Step 2: Correct the loop condition

   The loop should run while i < prefix.length() to access valid characters only.

3. Final Answer:

   The loop condition should be < instead of <= to avoid IndexOutOfBoundsException -> Option B

4. Quick Check:

   Loop index must be less than length [OK]

Hint: Loop index must be less than string length [OK]

Common Mistakes:

• Using <= instead of < in loop condition
• Incorrect character arithmetic
• Returning wrong boolean at the end

5. You want to find all words in a Trie that start with the prefix "app". Which approach correctly retrieves all such words?

hard

A. Use a hash map to store all words and filter those starting with "app"

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

C. Traverse the Trie to the node representing 'p' in "app", then perform a depth-first search collecting all words below

D. Insert "app" as a word and then search for it

Solution

1. Step 1: Locate the prefix node in the Trie

   Traverse the Trie following letters 'a', 'p', 'p' to reach the node representing the last 'p' in "app".

2. Step 2: Collect all words below this node

   Perform a depth-first search (DFS) from this node to find all complete words that start with "app".

3. Final Answer:

   Traverse the Trie to the node representing 'p' in "app", then perform a depth-first search collecting all words below -> Option C

4. Quick Check:

   Prefix node + DFS = all matching words [OK]

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

Common Mistakes:

• Checking all words instead of using Trie structure
• Using hash map instead of Trie traversal
• Confusing insertion with search