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

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of the postorder traversal?

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

Tree structure:
1
/ \
2 3
/ \
4 5

DSA Javascript

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

function postorder(root) {
  if (!root) return [];
  return [...postorder(root.left), ...postorder(root.right), root.value];
}

const root = new Node(1);
root.left = new Node(2);
root.right = new Node(3);
root.left.left = new Node(4);
root.left.right = new Node(5);

console.log(postorder(root));

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

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

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

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

💡 Hint

Remember postorder means visit left subtree, then right subtree, then the root node.

In postorder traversal, we visit left child, then right child, then the node itself. So for the tree, the order is 4 (left of 2), 5 (right of 2), 2 (root of left subtree), 3 (right child of root), then 1 (root).

❓ Predict Output

intermediate

What is the postorder traversal output for this tree?

Consider this binary tree:

10
/ \
20 30
/ \
40 50

What is the postorder traversal output (Left, Right, Root)?

DSA Javascript

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

function postorder(root) {
  if (!root) return [];
  return [...postorder(root.left), ...postorder(root.right), root.value];
}

const root = new Node(10);
root.left = new Node(20);
root.right = new Node(30);
root.right.left = new Node(40);
root.right.right = new Node(50);

console.log(postorder(root));

A [20, 40, 50, 30, 10]

B [10, 20, 30, 40, 50]

C [40, 50, 30, 20, 10]

D [20, 30, 40, 50, 10]

💡 Hint

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

The traversal visits 20 (left child), then 40 and 50 (left and right of 30), then 30, and finally 10 (root).

🧠 Conceptual

advanced

Which statement about postorder traversal is true?

Choose the correct statement about postorder traversal of a binary tree.

A Postorder traversal visits the root node before its children.

B Postorder traversal visits left subtree, then right subtree, then the root node.

C Postorder traversal visits nodes in the order: Right, Left, Root.

D Postorder traversal visits nodes in the order: Root, Left, Right.

💡 Hint

Think about when the root node is visited in postorder.

Postorder traversal always visits the left subtree first, then the right subtree, and finally the root node.

🔧 Debug

advanced

Find the error in this postorder traversal code

The following code is intended to perform a postorder traversal of a binary tree and return an array of node values. What is the error?

DSA Javascript

function postorder(root) {
  if (!root) return [];
  return [root.value, ...postorder(root.left), ...postorder(root.right)];
}

A There is no error; this is correct postorder traversal.

B The function will cause a runtime error because of incorrect spread syntax.

C The function will return an empty array for any input.

D The root node is visited before its children, so this is preorder traversal, not postorder.

💡 Hint

Check the order in which root and children are visited.

The code visits the root first, then left and right children, which is preorder traversal, not postorder.

🚀 Application

expert

How many nodes are visited before the root in postorder traversal?

In a postorder traversal of a binary tree with 7 nodes, how many nodes are visited before the root node is visited?

A 6

B 7

C 0

D Depends on the tree structure

💡 Hint

Postorder visits all children before the root.

Postorder traversal visits all nodes in the left and right subtrees before visiting the root node, so all other nodes (6) are visited first.