Count Total Nodes in Binary Tree in DSA Typescript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to return the total number of nodes in the binary tree.

DSA Typescript

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

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

A1

B0

Cnull

D-1

Attempts:

3 left

💡 Hint

Common Mistakes

Returning 1 instead of 0 for null nodes.

Returning null which causes errors.

When the node is null, it means no node exists, so count is 0.

2fill in blank

medium

Complete the code to check if the current node is null before counting.

DSA Typescript

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

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

Anull

Broot.left

Croot.right

Droot

Attempts:

3 left

💡 Hint

Common Mistakes

Checking left or right child instead of the current node.

Checking against a wrong variable.

We check if the current node root is null to stop recursion.

3fill in blank

hard

Fix the error in the recursive call to count left subtree nodes.

DSA Typescript

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

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

Aroot.right

Bnull

Croot.left

Droot

Attempts:

3 left

💡 Hint

Common Mistakes

Passing the whole root again causing infinite recursion.

Passing right child instead of left child.

To count left subtree nodes, we call countNodes(root.left).

4fill in blank

hard

Fill both blanks to complete the function that counts nodes using recursion.

DSA Typescript

function countNodes(root: TreeNode | null): number {
  if ([1] === null) return 0;
  return 1 + countNodes([2]) + countNodes(root.right);
}

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

Aroot

Broot.left

Croot.right

Dnull

Attempts:

3 left

💡 Hint

Common Mistakes

Checking left or right child instead of root for null.

Passing wrong child node in recursive call.

The base case checks if root is null, then recursively count left subtree with root.left.

5fill in blank

hard

Fill all three blanks to complete the recursive function counting all nodes in the binary tree.

DSA Typescript

function countNodes(root: TreeNode | null): number {
  if ([1] === null) return 0;
  return 1 + countNodes([2]) + countNodes([3]);
}

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

Aroot

Broot.left

Croot.right

Dnull

Attempts:

3 left

💡 Hint

Common Mistakes

Using null instead of root for base case.

Mixing left and right child nodes in recursive calls.

Check if root is null, then count nodes in left subtree root.left and right subtree root.right.