Tree Traversal Inorder Left Root Right in DSA Typescript - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the inorder traversal output of this binary tree?

Given the binary tree below, what is the output of inorder traversal (Left, Root, Right)?

DSA Typescript

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

const root = new Node(2, new Node(1), new Node(3));

function inorderTraversal(root: Node | null): number[] {
  if (!root) return [];
  return [...inorderTraversal(root.left), root.val, ...inorderTraversal(root.right)];
}

console.log(inorderTraversal(root));

A [1, 2, 3]

B [1, 3, 2]

C [3, 2, 1]

D [2, 1, 3]

💡 Hint

Remember inorder traversal visits left child first, then root, then right child.

Inorder traversal visits nodes in the order: left subtree, root, right subtree.

For the tree with root 2, left child 1, and right child 3, the order is 1, 2, 3.

❓ Predict Output

intermediate

What is the inorder traversal output of this unbalanced tree?

Given the binary tree below, what is the output of inorder traversal (Left, Root, Right)?

DSA Typescript

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

const root = new Node(4, new Node(2, new Node(1), new Node(3)), new Node(5));

function inorderTraversal(root: Node | null): number[] {
  if (!root) return [];
  return [...inorderTraversal(root.left), root.val, ...inorderTraversal(root.right)];
}

console.log(inorderTraversal(root));

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

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

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

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

💡 Hint

Traverse left subtree fully before root, then right subtree.

Inorder traversal visits left subtree nodes first, then root, then right subtree nodes.

For this tree, the order is 1, 2, 3, 4, 5.

🔧 Debug

advanced

Why does this inorder traversal code cause a runtime error?

Find the cause of the runtime error in this inorder traversal function.

DSA Typescript

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

function inorderTraversal(root: Node | null): number[] {
  if (root === null) return [];
  const left = inorderTraversal(root.left);
  const right = inorderTraversal(root.right);
  return left.concat([root.val]).concat(right);
}

const root = new Node(1);
console.log(inorderTraversal(root));

A No error, outputs [1]

B SyntaxError due to missing semicolon

C Stack overflow due to infinite recursion

D TypeError because concat expects an array but root.val is a number

💡 Hint

Check the types passed to concat method.

The concat method expects an array argument, but root.val is a number.

This causes a TypeError at runtime.

❓ Predict Output

advanced

What is the inorder traversal output of this tree with null children?

Given the binary tree below, what is the output of inorder traversal (Left, Root, Right)?

DSA Typescript

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

const root = new Node(10, null, new Node(20, new Node(15), null));

function inorderTraversal(root: Node | null): number[] {
  if (!root) return [];
  return [...inorderTraversal(root.left), root.val, ...inorderTraversal(root.right)];
}

console.log(inorderTraversal(root));

A [20, 15, 10]

B [10, 15, 20]

C [15, 20, 10]

D [10, 20, 15]

💡 Hint

Remember to visit left subtree before root, even if left is null.

Inorder traversal visits left subtree, root, then right subtree.

Here, left of root is null, so visit root 10, then traverse right subtree which visits 15 then 20.

🧠 Conceptual

expert

How many nodes are visited before the root in inorder traversal of a complete binary tree of height 3?

Consider a complete binary tree of height 3 (levels 0 to 3). How many nodes are visited before the root node during an inorder traversal?

A 4

B 6

C 7

D 3

💡 Hint

Count all nodes in the left subtree of the root.

In inorder traversal, all nodes in the left subtree are visited before the root.

A complete binary tree of height 3 has 7 nodes in the left subtree (levels 1 and 2 fully filled).