Trie Node Design and Initialization in DSA Python - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to initialize a TrieNode with an empty dictionary for children.

DSA Python

class TrieNode:
    def __init__(self):
        self.children = [1]
        self.is_end_of_word = False

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

A{}

B[]

Cset()

DNone

Attempts:

3 left

💡 Hint

Common Mistakes

Using a list or set instead of a dictionary for children.

The children of a TrieNode should be stored in a dictionary to map characters to child nodes. Using {} initializes an empty dictionary.

2fill in blank

medium

Complete the code to mark the node as the end of a word.

DSA Python

class TrieNode:
    def __init__(self):
        self.children = {}
        self.is_end_of_word = [1]

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

AFalse

BTrue

CNone

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Setting is_end_of_word to True by default.

When a TrieNode is created, it is not the end of a word by default, so is_end_of_word should be False.

3fill in blank

hard

Fix the error in the code to correctly initialize the children dictionary.

DSA Python

class TrieNode:
    def __init__(self):
        self.children = dict()
        self.is_end_of_word = False

node = TrieNode()
print(type(node.children).__name__ == [1])

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

Alist

Bset

Cdict

Dtuple

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing dict with list or set.

The children attribute is a dictionary, so type(node.children) should be dict.

4fill in blank

hard

Fill both blanks to add a child node for character 'a' and mark it as end of word.

DSA Python

node = TrieNode()
node.children[[1]] = TrieNode()
node.children['a'].is_end_of_word = [2]

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

A'a'

BTrue

CFalse

D'b'

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong character keys or wrong boolean values.

We add a child node for character 'a' and mark that child as the end of a word by setting is_end_of_word to True.

5fill in blank

hard

Fill all three blanks to check if a character exists in children and mark end of word.

DSA Python

node = TrieNode()
char = 'c'
if [1] in node.children:
    node.children[char].is_end_of_word = [2]
else:
    node.children[char] = TrieNode()
    node.children[char].is_end_of_word = [3]

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

Achar

BTrue

CFalse

D'c'

Attempts:

3 left

💡 Hint

Common Mistakes

Using the character literal instead of variable, or setting wrong boolean values.

We check if char is in children, mark existing child as end of word with True, else create new child and mark it as end of word also with True.

Practice

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

easy

A. To store the length of the longest word

B. To store children nodes representing characters and a flag for word end

C. To store the frequency of words in a dictionary

D. To store only the complete words in a list

Solution

1. Step 1: Understand Trie node components

   A Trie node typically holds links to child nodes for each character and a boolean flag to mark if a word ends here.

2. Step 2: Compare options with Trie node purpose

   Only To store children nodes representing characters and a flag for word end correctly describes storing children nodes and a word-end flag, which is essential for Trie functionality.

3. Final Answer:

   To store children nodes representing characters and a flag for word end -> Option B

4. Quick Check:

   Trie node = children + end flag [OK]

Hint: Trie nodes hold children and word-end flags [OK]

Common Mistakes:

• Thinking Trie nodes store whole words directly
• Confusing Trie nodes with hash maps
• Ignoring the word-end flag in nodes

2. Which of the following is the correct way to initialize a Trie node's children in Python for lowercase English letters?

easy

A. children = [0] * 26

B. children = {}

C. children = list('abcdefghijklmnopqrstuvwxyz')

D. children = [None] * 26

Solution

1. Step 1: Understand children initialization for fixed alphabet

   For 26 lowercase letters, children are usually a list of size 26 initialized with None to hold child nodes.

2. Step 2: Evaluate options

   children = [None] * 26 creates a list of 26 None values, suitable for storing child nodes. children = {} uses a dictionary, which is valid but not fixed size. children = [0] * 26 uses zeros, which are not node references. children = list('abcdefghijklmnopqrstuvwxyz') creates a list of characters, not nodes.

3. Final Answer:

   children = [None] * 26 -> Option D

4. Quick Check:

   Fixed-size list with None = children [OK]

Hint: Use list of None for fixed-size children array [OK]

Common Mistakes:

• Using zeros instead of None for children
• Using a string list instead of node references
• Confusing dictionary with fixed-size list

3. What will be the output of this code snippet?

class TrieNode:
    def __init__(self):
        self.children = [None] * 26
        self.is_end = False

node = TrieNode()
print(node.children[0])
print(node.is_end)

medium

A. 0\nFalse

B. None\nTrue

C. None\nFalse

D. Error

Solution

1. Step 1: Analyze TrieNode initialization

   The children list is initialized with 26 None values, so node.children[0] is None. The is_end flag is set to False.

2. Step 2: Check print outputs

   Printing node.children[0] outputs None, and printing node.is_end outputs False.

3. Final Answer:

   None\nFalse -> Option C

4. Quick Check:

   children[0] = None, is_end = False [OK]

Hint: Uninitialized children are None, is_end defaults False [OK]

Common Mistakes:

• Expecting 0 instead of None for children
• Assuming is_end defaults to True
• Thinking code raises error

4. Identify the error in this TrieNode initialization code:

class TrieNode:
    def __init__(self):
        self.children = [False] * 26
        self.is_end = False

medium

A. Children initialized with False instead of None

B. is_end should be True initially

C. Children list size should be 52 for uppercase and lowercase

D. Missing return statement in __init__

Solution

1. Step 1: Check children initialization

   Children should be initialized with None to indicate no child nodes yet. Using False is incorrect because False is a boolean, not a placeholder for a node.

2. Step 2: Verify other parts

   is_end is correctly set to False. The list size 26 is fine for lowercase letters. __init__ does not require a return statement.

3. Final Answer:

   Children initialized with False instead of None -> Option A

4. Quick Check:

   Children must be None, not False [OK]

Hint: Children must be None, not False [OK]

Common Mistakes:

• Confusing False with None for children
• Thinking __init__ needs return
• Assuming is_end should start True

5. You want to design a TrieNode that supports both lowercase and uppercase English letters. How should you initialize the children array?

hard

A. children = [None] * 52

B. children = [None] * 26

C. children = {}

D. children = list('abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ')

Solution

1. Step 1: Determine size for both cases

   Lowercase and uppercase English letters total 26 + 26 = 52 characters, so children array must have 52 slots.

2. Step 2: Choose correct initialization

   children = [None] * 52 creates a list of 52 None values, perfect for indexing each letter. children = [None] * 26 only supports 26 letters. children = {} uses a dictionary, which is flexible but not fixed size. children = list('abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ') creates a list of characters, not nodes.

3. Final Answer:

   children = [None] * 52 -> Option A

4. Quick Check:

   52 slots for uppercase + lowercase [OK]

Hint: Use 52 None slots for both letter cases [OK]

Common Mistakes:

• Using 26 slots for both cases
• Using dictionary instead of list
• Confusing list of chars with node references