Serialize and Deserialize Binary Tree in DSA Typescript - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Preorder Serialization

What is the output of the following TypeScript code that serializes a binary tree using preorder traversal with 'null' for empty nodes?

DSA Typescript

class TreeNode {
  val: number;
  left: TreeNode | null;
  right: TreeNode | null;
  constructor(val?: number, left?: TreeNode | null, right?: TreeNode | null) {
    this.val = val === undefined ? 0 : val;
    this.left = left === undefined ? null : left;
    this.right = right === undefined ? null : right;
  }
}

function serialize(root: TreeNode | null): string {
  if (!root) return 'null';
  return root.val + ',' + serialize(root.left) + ',' + serialize(root.right);
}

const tree = new TreeNode(1, new TreeNode(2), new TreeNode(3, new TreeNode(4), new TreeNode(5)));
console.log(serialize(tree));

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

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

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

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

💡 Hint

Think about preorder traversal: root, left subtree, then right subtree. Use 'null' for missing children.

The serialization uses preorder traversal. For each node, it records the value, then recursively serializes the left and right children. Null children are represented as 'null'. The output string is a comma-separated list of values and 'null's.

❓ Predict Output

intermediate

Result of Deserializing a Serialized Tree

Given the serialized string "1,2,null,null,3,4,null,null,5,null,null", what is the value of the root's right child's left child after deserializing with the code below?

DSA Typescript

class TreeNode {
  val: number;
  left: TreeNode | null;
  right: TreeNode | null;
  constructor(val?: number, left?: TreeNode | null, right?: TreeNode | null) {
    this.val = val === undefined ? 0 : val;
    this.left = left === undefined ? null : left;
    this.right = right === undefined ? null : right;
  }
}

function deserialize(data: string): TreeNode | null {
  const list = data.split(',');
  function helper(): TreeNode | null {
    if (list.length === 0) return null;
    const val = list.shift();
    if (val === 'null') return null;
    const node = new TreeNode(Number(val));
    node.left = helper();
    node.right = helper();
    return node;
  }
  return helper();
}

const root = deserialize("1,2,null,null,3,4,null,null,5,null,null");
console.log(root?.right?.left?.val);

A 4

B 5

C 3

D null

💡 Hint

Trace the deserialization step by step, matching nodes to values in preorder.

The deserialization reconstructs the tree in preorder. The root's right child is 3, and its left child is 4, so the output is 4.

🔧 Debug

advanced

Identify the Error in Serialization Code

What error does the following TypeScript serialization code produce when run on a non-empty tree?

DSA Typescript

function serialize(root: TreeNode | null): string {
  if (!root) return '';
  return root.val + ',' + serialize(root.left) + ',' + serialize(root.right);
}

const tree = new TreeNode(1, new TreeNode(2), null);
console.log(serialize(tree));

A SyntaxError due to missing return type

B The output string has extra commas and missing 'null' for empty nodes

C TypeError because root.val is undefined

D Runtime error due to infinite recursion

💡 Hint

Consider what happens when a child node is null and how it is represented in the output string.

The code returns an empty string for null nodes instead of 'null'. This causes missing markers for empty nodes and extra commas in the output string, which can cause problems during deserialization.

🚀 Application

advanced

Number of Nodes After Deserialization

After deserializing the string "1,2,null,null,3,null,4,null,null" using the standard preorder deserialization method, how many nodes does the resulting binary tree contain?

DSA Typescript

function deserialize(data: string): TreeNode | null {
  const list = data.split(',');
  function helper(): TreeNode | null {
    if (list.length === 0) return null;
    const val = list.shift();
    if (val === 'null') return null;
    const node = new TreeNode(Number(val));
    node.left = helper();
    node.right = helper();
    return node;
  }
  return helper();
}

const root = deserialize("1,2,null,null,3,null,4,null,null");

A 5

B 3

C 6

D 4

💡 Hint

Count nodes by tracing the preorder deserialization and ignoring 'null' markers.

The tree has nodes with values 1, 2, 3, and 4. Nulls mark missing children. So total nodes are 4.

🧠 Conceptual

expert

Why Use 'null' Markers in Serialization?

Why is it important to include explicit 'null' markers for empty nodes when serializing a binary tree using preorder traversal?

A To preserve the exact tree structure including missing children during deserialization

B To reduce the size of the serialized string by skipping empty nodes

C To make the serialization compatible with inorder traversal

D To allow the tree to be serialized without recursion

💡 Hint

Think about what happens if you omit empty nodes in the serialized data.

Null markers are needed to know where children are missing, so the tree can be rebuilt exactly as it was. Without them, the structure is lost.