Tree Traversal Preorder Root Left Right in DSA Javascript - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this preorder traversal?

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

DSA Javascript

class Node {
  constructor(value, left = null, right = null) {
    this.value = value;
    this.left = left;
    this.right = right;
  }
}

const root = new Node(10,
  new Node(5,
    new Node(3),
    new Node(7)
  ),
  new Node(15,
    null,
    new Node(18)
  )
);

function preorder(node, result = []) {
  if (!node) return result;
  result.push(node.value);
  preorder(node.left, result);
  preorder(node.right, result);
  return result;
}

console.log(preorder(root).join(' -> ') + ' -> null');

A 10 -> 5 -> 3 -> 7 -> 15 -> 18 -> null

B 3 -> 5 -> 7 -> 10 -> 15 -> 18 -> null

C 10 -> 15 -> 18 -> 5 -> 3 -> 7 -> null

D 5 -> 3 -> 7 -> 10 -> 15 -> 18 -> null

💡 Hint

Remember preorder visits root first, then left subtree, then right subtree.

Preorder traversal visits nodes in the order: root, left child, then right child.

Starting at 10 (root), then left subtree 5, then 3 and 7, then right subtree 15 and 18.

❓ Predict Output

intermediate

Output of preorder traversal on a single-node tree

What is the output of the preorder traversal on this tree with only one node?

DSA Javascript

class Node {
  constructor(value, left = null, right = null) {
    this.value = value;
    this.left = left;
    this.right = right;
  }
}

const root = new Node(42);

function preorder(node, result = []) {
  if (!node) return result;
  result.push(node.value);
  preorder(node.left, result);
  preorder(node.right, result);
  return result;
}

console.log(preorder(root).join(' -> ') + ' -> null');

A 42 -> 42 -> null

B 42

C 42 -> null

D null

💡 Hint

Preorder always includes the root node first.

With only one node, preorder traversal visits that node and then ends.

🔧 Debug

advanced

Identify the error in this preorder traversal function

What error will this preorder traversal code produce when run?

DSA Javascript

function preorder(node, result = []) {
  if (node == null) return;
  result.push(node.value);
  preorder(node.left, result);
  preorder(node.right, result);
  return result;
}

const root = null;
console.log(preorder(root));

A TypeError

B [1]

C null

D undefined

💡 Hint

Check what happens when the function returns without a value.

The function returns undefined when node is null because it returns without a value.

This causes the final console.log to print undefined.

❓ Predict Output

advanced

Preorder traversal output with unbalanced tree

What is the output of the preorder traversal for this unbalanced tree?

DSA Javascript

class Node {
  constructor(value, left = null, right = null) {
    this.value = value;
    this.left = left;
    this.right = right;
  }
}

const root = new Node(1,
  new Node(2,
    new Node(4),
    null
  ),
  new Node(3,
    null,
    new Node(5,
      null,
      new Node(6)
    )
  )
);

function preorder(node, result = []) {
  if (!node) return result;
  result.push(node.value);
  preorder(node.left, result);
  preorder(node.right, result);
  return result;
}

console.log(preorder(root).join(' -> ') + ' -> null');

A 1 -> 3 -> 5 -> 6 -> 2 -> 4 -> null

B 1 -> 2 -> 4 -> 3 -> 5 -> 6 -> null

C 4 -> 2 -> 1 -> 3 -> 5 -> 6 -> null

D 1 -> 2 -> 3 -> 4 -> 5 -> 6 -> null

💡 Hint

Preorder visits root first, then left subtree, then right subtree.

Start at 1, then left subtree 2 and 4, then right subtree 3, 5, and 6.

🧠 Conceptual

expert

How many nodes are visited in preorder traversal of a full binary tree with height 3?

A full binary tree is a tree where every node has 0 or 2 children. If the tree has height 3 (root is height 1), how many nodes will the preorder traversal visit?

A 7

B 15

C 3

D 8

💡 Hint

Number of nodes in a full binary tree of height h is 2^h - 1.

A full binary tree of height 3 has 2^3 - 1 = 7 nodes.

Preorder traversal visits all nodes, so it visits 7 nodes.