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

Recall & Review

beginner

What does it mean to serialize a binary tree?

Serialization means converting a binary tree into a string or list format so it can be saved or sent somewhere and later rebuilt exactly as it was.

beginner

What is the main goal of deserialization in binary trees?

Deserialization means taking the serialized string or list and rebuilding the original binary tree structure from it.

beginner

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

We use markers to show where a node does not have a child. This helps keep the tree shape when we rebuild it during deserialization.

intermediate

Which tree traversal methods are commonly used for serialization?

Preorder (root-left-right) and level-order (breadth-first) traversals are commonly used because they capture the tree structure well.

intermediate

What is a simple approach to deserialize a binary tree from a preorder serialized string?

Read the values one by one. If the value is a marker (like 'null'), return no node. Otherwise, create a node and recursively build its left and right children.

What is the purpose of serialization in binary trees?

A To delete the tree nodes

B To convert the tree into a format that can be stored or sent

C To balance the tree

D To sort the tree nodes

Serialization converts the tree into a storable or transferable format.

Which marker is commonly used to represent a missing child in serialization?

A null

B 0

C 1

D root

The 'null' marker shows where a node does not have a child.

Which traversal is often used for serializing a binary tree?

A Inorder

B Postorder

C Preorder

D Random

Preorder traversal (root-left-right) is commonly used for serialization.

During deserialization, what do you do when you encounter a 'null' marker?

A Create a new node

B Restart the process

C Add a leaf node

D Skip and return no node

A 'null' marker means no node exists there, so return nothing.

What data structure can help in level-order serialization?

A Queue

B Set

C Stack

D Map

A queue helps process nodes level by level in level-order traversal.

Explain how you would serialize a binary tree using preorder traversal.

Think about visiting the root first, then children, and marking empty spots.

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

Imagine rebuilding the tree by reading the string from start to end.

Practice

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

easy

A. Sort the nodes of a binary tree in ascending order

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

C. Calculate the height of a binary tree

D. Delete all nodes in a binary 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 binary tree -> Option B

4. Quick Check:

   Serialize/deserialize = convert tree to/from string [OK]

Hint: Think of saving and loading a tree as text [OK]

Common Mistakes:

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

2. Which of the following is the correct way to declare a function in JavaScript for serializing a binary tree?

easy

A. function: serialize(root) { /* code */ }

B. serialize function(root) { /* code */ }

C. def serialize(root): { /* code */ }

D. function serialize(root) { /* code */ }

Solution

1. Step 1: Recall JavaScript function syntax

   In JavaScript, functions are declared using the keyword function followed by the name and parameters.

2. Step 2: Check each option

   function serialize(root) { /* code */ } uses correct syntax. Options A, B, and D use invalid or other language syntax.

3. Final Answer:

   function serialize(root) { /* code */ } -> Option D

4. Quick Check:

   JS function declaration = function name(params) [OK]

Hint: Use 'function name(params)' to declare in JS [OK]

Common Mistakes:

• Using Python or other language syntax
• Missing the 'function' keyword
• Using colons or wrong punctuation

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

class TreeNode {
  constructor(val, left = null, right = null) {
    this.val = val;
    this.left = left;
    this.right = right;
  }
}

function serialize(root) {
  if (!root) return '#';
  return root.val + ',' + serialize(root.left) + ',' + serialize(root.right);
}

const root = new TreeNode(1, new TreeNode(2), new TreeNode(3));
console.log(serialize(root));

medium

A. "1,2,#,#,3,#,#"

B. "1,2,3,#,#,#,#"

C. "1,#,2,#,3,#,#"

D. "#,#,1,2,3"

Solution

1. Step 1: Understand the serialize function

   The function uses preorder traversal: root, left, right. Null nodes are represented by '#'.

2. Step 2: Trace the tree serialization

   Root is 1, left child 2 (with no children), right child 3 (with no children). So serialization is "1,2,#,#,3,#,#".

3. Final Answer:

   "1,2,#,#,3,#,#" -> Option A

4. Quick Check:

   Preorder with '#' for null = "1,2,#,#,3,#,#" [OK]

Hint: Preorder traversal with '#' for null nodes [OK]

Common Mistakes:

• Confusing preorder with inorder or postorder
• Forgetting to add '#' for null children
• Mixing order of nodes in output

4. Identify the error in the following deserialize function for a binary tree:

function deserialize(data) {
  const values = data.split(',');
  function build() {
    if (values.length === 0) return null;
    const val = values.shift();
    if (val === '#') return null;
    const node = new TreeNode(val);
    node.left = build();
    node.right = build();
    return node;
  }
  return build();
}

medium

A. The TreeNode constructor should convert val to number

B. The function does not handle empty input string

C. The values array should be reversed before processing

D. The function should use pop() instead of shift()

Solution

1. Step 1: Analyze the deserialize function

   The function splits the string and uses shift() to get values in order, which is correct.

2. Step 2: Check TreeNode value type

   Values from split are strings. If TreeNode expects numbers, val should be converted using Number(val) or +val.

3. Final Answer:

   The TreeNode constructor should convert val to number -> Option A

4. Quick Check:

   Convert string to number for node values [OK]

Hint: Convert string values to numbers when creating nodes [OK]

Common Mistakes:

• Ignoring type conversion of node values
• Reversing array unnecessarily
• Using pop() breaks preorder order

5. Given the serialized string "1,2,#,#,3,4,#,#,5,#,#", which of the following binary trees is the correct deserialized structure?

hard

A. Root 1 with left child 3, right child 2 with children 4 and 5

B. Root 1 with left child 2 with children 4 and 5, right child 3 (no children)

C. Root 1 with left child 2 (no children), right child 3 with left 4 and right 5

D. Root 1 with left child 4, right child 5 with children 2 and 3

Solution

1. Step 1: Understand the serialized string format

   The string uses preorder traversal with '#' for nulls: root, left subtree, right subtree.

2. Step 2: Trace the tree structure

   Root is 1, left child 2 with no children (#,#), right child 3 with left child 4 (no children) and right child 5 (no children).

3. Final Answer:

   Root 1 with left child 2 (no children), right child 3 with left 4 and right 5 -> Option C

4. Quick Check:

   Preorder string matches tree structure in Root 1 with left child 2 (no children), right child 3 with left 4 and right 5 [OK]

Hint: Draw preorder steps to visualize tree structure [OK]

Common Mistakes:

• Mixing left and right children
• Ignoring '#' as null nodes
• Misreading preorder order