Serialize and Deserialize Binary Tree in DSA Python - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a new node with the given value.

DSA Python

class TreeNode:
    def __init__(self, val=0, left=None, right=None):
        self.val = [1]
        self.left = left
        self.right = right

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

ANone

B0

Cval

Dself

Attempts:

3 left

💡 Hint

Common Mistakes

Using None instead of val for the node's value.

Using self instead of val.

The constructor assigns the passed value val to the node's val attribute.

2fill in blank

medium

Complete the code to append the serialized value to the result list.

DSA Python

def serialize(root):
    result = []
    def helper(node):
        if not node:
            result.append('null')
            return
        result.append(str(node.[1]))
        helper(node.left)
        helper(node.right)
    helper(root)
    return ','.join(result)

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

Aleft

Bval

Cright

Ddata

Attempts:

3 left

💡 Hint

Common Mistakes

Appending node.left or node.right instead of node.val.

Appending node.data which does not exist.

We append the string form of the node's value (val) to the result list during serialization.

3fill in blank

hard

Fix the error in the deserialization code to correctly convert the string value to an integer.

DSA Python

def deserialize(data):
    values = data.split(',')
    def helper():
        val = values.pop(0)
        if val == 'null':
            return None
        node = TreeNode([1])
        node.left = helper()
        node.right = helper()
        return node
    return helper()

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

Aint(val)

Bval

Cstr(val)

Dfloat(val)

Attempts:

3 left

💡 Hint

Common Mistakes

Using val directly without conversion causes type errors.

Using float conversion instead of int.

The serialized values are strings, so we convert val to an integer before creating the node.

4fill in blank

hard

Fill both blanks to complete the serialize function using preorder traversal.

DSA Python

def serialize(root):
    result = []
    def helper(node):
        if node is None:
            result.append('null')
            return
        result.append(str(node.[1]))
        helper(node.[2])
        helper(node.right)
    helper(root)
    return ','.join(result)

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

Aval

Bleft

Cright

Ddata

Attempts:

3 left

💡 Hint

Common Mistakes

Using right child before left child breaks preorder order.

Appending wrong attributes like data.

We append the node's value (val) and recursively call helper on the left child (left) first for preorder traversal.

5fill in blank

hard

Fill all three blanks to complete the deserialize function that rebuilds the tree.

DSA Python

def deserialize(data):
    values = data.split(',')
    def helper():
        val = values.pop(0)
        if val == 'null':
            return [1]
        node = TreeNode([2])
        node.left = helper()
        node.right = helper()
        return node
    return helper()

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

ANone

Bint(val)

Cval

DTreeNode(val)

Attempts:

3 left

💡 Hint

Common Mistakes

Returning 'null' string instead of None.

Not converting val to int causes errors.

Return None for 'null' values, convert val to int for node creation, and use None for missing children.

Practice

1. What is the main purpose of serialize and deserialize functions in the context of a binary tree?

easy

A. Delete all nodes in a binary tree

B. Sort the nodes of a binary tree

C. Calculate the height of a binary tree

D. Convert a binary tree to a string and back to a tree

Solution

1. Step 1: Understand serialization

   Serialization means converting a data structure like a binary tree into a string format that can be stored or transmitted.

2. Step 2: Understand deserialization

   Deserialization means converting the string back into the original binary tree structure.

3. Final Answer:

   Convert a binary tree to a string and back to a tree -> Option D

4. Quick Check:

   Serialize + Deserialize = Convert tree to string and back [OK]

Hint: Serialization means tree to string, deserialization reverses it [OK]

Common Mistakes:

• Confusing serialization with sorting
• Thinking it deletes nodes
• Mixing serialization with height calculation

2. Which of the following is the correct way to represent a null node during serialization in a binary tree using preorder traversal?

easy

A. Use a special marker like '#' or 'null'

B. Skip the null node completely

C. Use an empty string ''

D. Use the value 0

Solution

1. Step 1: Understand null node representation

   During serialization, null nodes must be marked explicitly to preserve tree structure.

2. Step 2: Choose correct marker

   Common practice is to use a special marker like '#' or 'null' to represent null nodes.

3. Final Answer:

   Use a special marker like '#' or 'null' -> Option A

4. Quick Check:

   Null nodes marked with '#' or 'null' [OK]

Hint: Mark null nodes explicitly with '#' or 'null' [OK]

Common Mistakes:

• Skipping null nodes loses tree shape
• Using empty string confuses parsing
• Using 0 can be mistaken for a valid node

3. Given the following serialized string of a binary tree using preorder with '#' as null marker: 1,2,#,#,3,4,#,#,5,#,#, what is the inorder traversal of the deserialized tree?

medium

A. [2, 4, 1, 3, 5]

B. [1, 2, 3, 4, 5]

C. [2, 1, 4, 3, 5]

D. [1, 3, 2, 4, 5]

Solution

1. Step 1: Reconstruct the tree from serialization

   The string represents preorder: root=1, left=2, left's children null, right=3, left=4, right=5.

2. Step 2: Perform inorder traversal (left, root, right)

   Inorder: left subtree (2), root (1), left subtree of right child (4), root of right child (3), right subtree (5) -> [2, 1, 4, 3, 5]

3. Final Answer:

   [2, 1, 4, 3, 5] -> Option C

4. Quick Check:

   Inorder of deserialized tree = [2, 1, 4, 3, 5] [OK]

Hint: Draw tree from preorder, then do inorder traversal [OK]

Common Mistakes:

• Mixing preorder and inorder orders
• Ignoring null markers during reconstruction
• Swapping left and right children

4. Consider this buggy deserialize function snippet for a binary tree serialized with preorder and '#' as null marker:

def deserialize(data):
    def helper():
        val = next(vals)
        if val == '#':
            return None
        node = TreeNode(int(val))
        node.left = helper()
        node.right = helper()
        return node
    vals = iter(data.split(','))
    return helper()

What is the likely cause of a StopIteration error during deserialization?

medium

A. The helper function does not return the root node

B. The input string is missing some null markers

C. The split method is used incorrectly

D. The TreeNode constructor is called with a string instead of int

Solution

1. Step 1: Understand StopIteration cause

   StopIteration occurs when iterator runs out of elements unexpectedly.

2. Step 2: Check input data correctness

   If null markers '#' are missing, helper calls next(vals) more times than available, causing StopIteration.

3. Final Answer:

   The input string is missing some null markers -> Option B

4. Quick Check:

   Missing '#' causes StopIteration [OK]

Hint: Check if all null nodes are marked to avoid StopIteration [OK]

Common Mistakes:

• Thinking helper must return root explicitly
• Blaming split method for StopIteration
• Confusing int conversion error with StopIteration

5. You want to serialize a binary tree and then deserialize it back. However, the tree can contain negative and zero values. Which serialization approach below correctly handles all integer values including zero and negative numbers without confusion?

hard

A. Serialize using preorder with comma-separated values and 'null' for null nodes

B. Serialize using comma-separated values but skip null nodes

C. Serialize using preorder with space-separated values and '#' for null nodes

D. Serialize using preorder with comma-separated values and '0' for null nodes

Solution

1. Step 1: Identify safe null marker

   Using '0' as null marker is unsafe because zero can be a valid node value.

2. Step 2: Choose clear null marker and separator

   Using 'null' as null marker with comma separator clearly distinguishes null nodes and supports negative and zero values.

3. Final Answer:

   Serialize using preorder with comma-separated values and 'null' for null nodes -> Option A

4. Quick Check:

   'null' marker avoids confusion with zero or negative values [OK]

Hint: Use 'null' marker, not '0', to handle zero and negatives [OK]

Common Mistakes:

• Using '0' as null marker causes confusion
• Skipping null nodes loses tree shape
• Using space separator can cause parsing issues