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Cursor·Software Engineer·Technical Phone Screen·Senior

SeniorPrefer not to say
Jun 2026Remote

Summary

Cursor SE interview had me implementing a Merkle tree from scratch, which sounds manageable until you're actually doing it under pressure. Pretty technical round, no fluff.

Questions Asked (1)

Q1

Implement a Merkle tree from a list of data blocks, supporting root hash exposure, inclusion proof generation, and proof verification without the full tree. Also discuss time complexity and real-world applications.

Algorithms & Data StructuresSystem DesignTechnical Trade-offs
Author's notes

This was the whole interview basically.

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements and edge cases, then design the Merkle tree with a clear hashing scheme (e.g., SHA-256) and define node structure. Implement the core functions—build tree, get root, generate proof, verify proof—and analyze time/space complexity. Finally, discuss real-world applications like blockchain and distributed systems, highlighting trade-offs.

Pro tip: Mention that proof verification only requires the root hash and the proof, not the full tree, and emphasize the logarithmic proof size. Also, discuss how to handle odd numbers of nodes (e.g., duplicate last node) to show attention to detail.

1. Clarify requirements and edge cases

Ask about data block size, hash function, handling of empty input, odd number of leaves, and whether the tree should be balanced. Confirm that proof verification should work without the full tree.

2. Design the Merkle tree structure

Define leaf nodes as hashes of data blocks, internal nodes as hashes of concatenated child hashes. Decide on a consistent hashing scheme (e.g., SHA-256) and how to handle odd nodes (e.g., promote or duplicate).

3. Implement core operations

Write functions to build the tree from a list of blocks, compute the root hash, generate an inclusion proof for a given block (list of sibling hashes), and verify a proof given a root hash and block.

4. Analyze complexity and trade-offs

Discuss time complexity: O(n) to build, O(log n) to generate/verify proof. Space: O(n) for tree, O(log n) for proof. Mention trade-offs like using a balanced tree vs. other structures.

5. Discuss real-world applications

Explain how Merkle trees are used in blockchain (e.g., Bitcoin), distributed systems (e.g., IPFS, Cassandra), and version control (e.g., Git) for efficient data verification and integrity.

Key Points to Mention

  • Hash function choice (e.g., SHA-256) and its properties (collision resistance, preimage resistance).
  • Proof generation and verification algorithm: sibling hashes combined with the target hash to recompute root.
  • Time complexity: O(n) build, O(log n) proof generation and verification; space complexity: O(n) tree, O(log n) proof.
  • Handling edge cases: empty input, single block, odd number of leaves (e.g., duplicate last hash).
  • Real-world applications: blockchain (Bitcoin, Ethereum), distributed storage (IPFS), databases (Cassandra), and version control (Git).
  • Security considerations: second preimage attacks and mitigation (e.g., prefixing leaf hashes with 0x00 and internal with 0x01).

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