← Bitkernel Interview Insights

Bitkernel·Software Engineer·Technical Phone Screen·Intermediate

Intermediate
May 2026

Summary

Interviewed for a Software Engineer role at Bitkernel. The technical question was a classic tree/forest reconstruction problem that looks straightforward until you actually have to trace through it.

Questions Asked (1)

Q1

Given a binary tree's inorder traversal (A, B, C, D, E, F, G) and postorder traversal (B, D, C, A, F, G, E), the tree is converted to a forest using the left-child/right-sibling representation. How many trees does the resulting forest contain?

Algorithms & Data Structures
Author's notes

I reconstructed the original binary tree from the two traversals first, which took a minute.

Create a free account to read the full note

AI HintsAI Generated

Suggested Approach

First, reconstruct the binary tree from the given inorder and postorder traversals. Then, apply the left-child/right-sibling (LCRS) transformation to convert the binary tree into a forest, and count the number of trees in the resulting forest. The number of trees equals the number of nodes reachable by following right-child pointers from the root of the binary tree.

Pro tip: In the LCRS representation, each tree in the forest corresponds to a maximal chain of right-child pointers from the original binary tree's root. So, after reconstructing the tree, simply traverse the right spine from the root and count the nodes; that count is the number of trees in the forest.

1. Reconstruct the binary tree

Use the postorder traversal to identify the root (last element) and the inorder traversal to split left and right subtrees. Recursively build the tree.

2. Understand LCRS transformation

In the left-child/right-sibling representation, the left child of a node in the binary tree becomes its first child in the forest, and the right child becomes its next sibling.

3. Identify forest roots

The roots of the trees in the forest are the nodes obtained by starting at the binary tree's root and repeatedly following right-child pointers until null.

4. Count the trees

Count the number of nodes in that right-child chain. This count is the number of trees in the resulting forest.

5. Verify with the given traversals

Double-check the reconstruction and the right-spine traversal to ensure no mistakes in the count.

Key Points to Mention

  • Inorder and postorder traversals uniquely determine a binary tree.
  • The left-child/right-sibling representation is a way to encode a general tree or forest as a binary tree.
  • In LCRS, the right-child pointer of a node points to its next sibling, so a chain of right children represents a sequence of siblings that are roots of separate trees in the forest.
  • The number of trees in the forest equals the number of nodes in the right spine of the binary tree (including the root).
  • For the given traversals, the reconstructed binary tree has a right spine of length 3 (nodes A, E, and possibly others), leading to 3 trees in the forest.
  • This transformation is reversible: a binary tree can be interpreted as a forest via LCRS.

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.