Serialize and Deserialize Binary Tree in DSA Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does it mean to serialize a binary tree?

Serialization means converting the binary tree into a string or list format that can be stored or transmitted easily, preserving the tree structure.

beginner

What is the purpose of deserialization in binary trees?

Deserialization means converting the serialized string or list back into the original binary tree structure with the same node values and connections.

beginner

Why do we use a special marker like 'null' or '#' during serialization?

We use a special marker to represent empty or missing child nodes so that the tree structure can be fully reconstructed during deserialization.

intermediate

Which tree traversal method is commonly used for serialization and why?

Preorder traversal is commonly used because it visits the root first, then left and right subtrees, allowing us to record the structure and values in order.

intermediate

What is the time complexity of serializing and deserializing a binary tree with n nodes?

Both serialization and deserialization take O(n) time because each node is visited once during the process.

What does the 'null' marker represent in serialized binary tree data?

A The root node

B A missing child node

C A leaf node

D A duplicate node

The 'null' marker shows where a child node is missing, helping to keep the tree structure intact.

Which traversal order is best suited for serializing a binary tree to preserve structure?

A Preorder

B Inorder

C Postorder

D Level order

Preorder traversal records the root before its children, making it easier to reconstruct the tree.

What is the main goal of deserializing a binary tree?

A To sort the tree nodes

B To convert a tree into a string

C To count the number of nodes

D To rebuild the original tree from serialized data

Deserialization rebuilds the original tree structure from the serialized string or list.

If a binary tree has 7 nodes, how many 'null' markers will appear in its serialized form?

A 8

B 14

C 6

D 7

A binary tree with n nodes has n+1 null pointers, so 7 nodes have 8 null markers.

What is the time complexity to serialize and deserialize a binary tree with n nodes?

A O(log n)

B O(n^2)

C O(n)

D O(1)

Both serialization and deserialization visit each node once, so the time complexity is O(n).

Explain how preorder traversal helps in serializing and deserializing a binary tree.

Think about the order nodes are saved and restored.

Describe the steps to deserialize a binary tree from a serialized string with null markers.

Focus on how recursion helps rebuild the tree.

Practice

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

easy

A. Find the height of the binary tree

B. Sort the binary tree nodes in ascending order

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

D. Delete all nodes in the binary tree

Solution

1. Step 1: Understand serialization

   Serialization means converting a binary tree into a string format that can be stored or sent.

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 binary tree -> Option C

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 deserialization deletes nodes
• Mixing serialization with tree height calculation

2. Which of the following TypeScript function signatures correctly represents a deserialize function for a binary tree?

easy

A. function deserialize(data: string): TreeNode | null

B. function deserialize(data: number): TreeNode

C. function deserialize(tree: TreeNode): string

D. function deserialize(): void

Solution

1. Step 1: Identify input and output types

   Deserialize takes a string (serialized data) and returns a TreeNode or null if empty.

2. Step 2: Match correct signature

   function deserialize(data: string): TreeNode | null matches input as string and output as TreeNode or null.

3. Final Answer:

   function deserialize(data: string): TreeNode | null -> Option A

4. Quick Check:

   Deserialize input string, output tree node [OK]

Hint: Deserialize input is string, output is tree node or null [OK]

Common Mistakes:

• Using number instead of string as input
• Returning string instead of tree node
• No input or void return type

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

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

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

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

Solution

1. Step 1: Reconstruct tree from preorder string

   Preorder: 1 (root), 2 (left), #,# (2's children null), 3 (right), 4 (3's left), #,# (4's children null), 5 (3's right), #,# (5's children null).

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

   Inorder: left subtree of 1 is 2, root 1, left subtree of 3 is 4, root 3, right subtree 5. So inorder is [2, 1, 4, 3, 5].

3. Final Answer:

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

4. Quick Check:

   Preorder string to tree, inorder traversal = [2,1,4,3,5] [OK]

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

Common Mistakes:

• Confusing preorder with inorder
• Misplacing null markers
• Incorrect traversal order

4. Consider this deserialize function snippet in TypeScript:

function deserialize(data: string): TreeNode | null {
  const values = data.split(",");
  function build(): TreeNode | null {
    if (!values.length) return null;
    const val = values.shift();
    if (val === "#") return null;
    const node = new TreeNode(parseInt(val));
    node.left = build();
    node.right = build();
    return node;
  }
  return build();
}

What is the bug in this code if the input string is empty?

medium

A. It returns null correctly, no bug

B. parseInt throws error on undefined

C. The function build never terminates

D. parseInt('') returns NaN, creating invalid TreeNode

Solution

1. Step 1: Check behavior on empty input

   If data is empty string, values = [""], so values.length is 1, not zero.

2. Step 2: Analyze values.shift() and val check

   values.shift() returns "" (empty string), val === "#" is false, parseInt("") returns NaN, which may cause issues when creating TreeNode.

3. Step 3: Identify actual bug

   parseInt("") returns NaN, so TreeNode is created with NaN value, which is likely unintended.

4. Final Answer:

   parseInt('') returns NaN, creating invalid TreeNode -> Option D

5. Quick Check:

   Empty input leads to undefined or NaN node value [OK]

Hint: Empty string splits to [''], not empty array; check val before parseInt [OK]

Common Mistakes:

• Assuming empty string splits to empty array
• Ignoring parseInt('') returns NaN
• Thinking function never ends

5. You want to serialize a binary tree using level-order traversal (BFS) with 'null' for missing nodes. Which of these serialized strings correctly represents the tree:

Tree:
1
/ \ 2 3
/ \ 4 5

Options:

hard

A. "1,2,3,null,null,4,5,null,null,null,null"

B. "1,2,3,4,5,null,null,null,null,null,null"

C. "1,3,2,null,4,5,null,null,null,null,null"

D. "1,2,3,4,null,5,null,null,null,null,null"

Solution

1. Step 1: Understand level-order traversal

   Level order visits nodes level by level: root, then children left to right, including nulls for missing nodes.

2. Step 2: Write level order for given tree

   Level 1: 1
   Level 2: 2, 3
   Level 3: null, null (2's children), 4, 5 (3's children)
   Level 4: nulls for 4 and 5's children
   So serialized string is "1,2,3,null,null,4,5,null,null,null,null".

3. Final Answer:

   "1,2,3,null,null,4,5,null,null,null,null" -> Option A

4. Quick Check:

   Level order with nulls matches "1,2,3,null,null,4,5,null,null,null,null" [OK]

Hint: Level order includes nulls for missing children [OK]

Common Mistakes:

• Mixing preorder with level order
• Omitting null placeholders
• Swapping left and right children