Count Total Nodes in Binary Tree in DSA Typescript - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Node Count in Simple Binary Tree

What is the output of the following TypeScript code that counts total nodes in a binary tree?

DSA Typescript

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

function countNodes(root: TreeNode | null): number {
  if (root === null) return 0;
  return 1 + countNodes(root.left) + countNodes(root.right);
}

const tree = new TreeNode(1, new TreeNode(2), new TreeNode(3));
console.log(countNodes(tree));

A 0

B 2

C 1

D 3

💡 Hint

Count the root node plus nodes in left and right subtrees.

The tree has root node 1, left child 2, and right child 3, total 3 nodes.

❓ Predict Output

intermediate

Count Nodes in Unbalanced Binary Tree

What will be printed after running this code that counts nodes in an unbalanced binary tree?

DSA Typescript

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

function countNodes(root: TreeNode | null): number {
  if (root === null) return 0;
  return 1 + countNodes(root.left) + countNodes(root.right);
}

const tree = new TreeNode(10, new TreeNode(5, new TreeNode(3)), null);
console.log(countNodes(tree));

A 1

B 3

C 2

D 0

💡 Hint

Count all nodes including left subtree nodes.

The tree has nodes 10, 5, and 3, total 3 nodes.

🔧 Debug

advanced

Identify the Error in Node Counting Function

What error will this TypeScript code produce when counting nodes in a binary tree?

DSA Typescript

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

function countNodes(root: TreeNode | null): number {
  if (root === null) return 0;
  return 1 + countNodes(root.left) - countNodes(root.right);
}

const tree = new TreeNode(1, new TreeNode(2), new TreeNode(3));
console.log(countNodes(tree));

A SyntaxError

B 2

C 1

D 0

💡 Hint

Check the operator used between recursive calls.

The code subtracts right subtree count instead of adding: 1 + 1 - 1 = 1 (logical error, outputs 1 instead of 3).

❓ Predict Output

advanced

Count Nodes in Large Balanced Binary Tree

What is the output of this code counting nodes in a balanced binary tree of height 3?

DSA Typescript

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

function countNodes(root: TreeNode | null): number {
  if (root === null) return 0;
  return 1 + countNodes(root.left) + countNodes(root.right);
}

const tree = new TreeNode(1,
  new TreeNode(2, new TreeNode(4), new TreeNode(5)),
  new TreeNode(3, new TreeNode(6), new TreeNode(7))
);
console.log(countNodes(tree));

A 7

B 6

C 8

D 5

💡 Hint

Count all nodes in a full binary tree with 3 levels.

A full binary tree with height 3 has 7 nodes total.

🧠 Conceptual

expert

Time Complexity of Counting Nodes in Binary Tree

What is the time complexity of the recursive function that counts total nodes in a binary tree with n nodes?

A O(n)

B O(log n)

C O(n log n)

D O(1)

💡 Hint

Consider how many nodes the function visits.

The function visits each node once, so time complexity is linear O(n).