Snowflake·Software Engineer·Technical Phone Screen·Senior

Senior

Jun 2026

Summary

Snowflake software engineer interview with a meaty tree problem that had more moving parts than I expected. The question looked like a straightforward traversal until they started asking about overflow handling and serialization helpers on top of it.

Questions Asked (1)

Given two complete binary trees with identical structure, replace every node value in the second tree with the sum of all values in the corresponding subtree of the first tree. Implement this in O(n) time and O(h) extra space, and also write helpers to build a tree from a level-order array and serialize it back to an array for verification.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

I went straight for a recursive post-order traversal and got the core transformation working pretty quickly.

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AI HintsAI Generated

Suggested Approach

Use a simultaneous recursive traversal of both trees, computing subtree sums from the first tree and assigning them to the corresponding nodes in the second tree. This achieves O(n) time and O(h) space, where h is the tree height. Then implement level-order build and serialize helpers for testing.

Pro tip: Clarify that the trees are complete and have identical structure, so you can safely assume corresponding nodes exist; this simplifies the recursion and avoids null checks. Also, mention that the O(h) space comes from the recursion stack, and for a complete tree h = O(log n), so it's very efficient.

1. Understand the problem and constraints

Restate the problem: given two complete binary trees with identical structure, replace each node's value in the second tree with the sum of the corresponding subtree in the first tree. Note the required O(n) time and O(h) extra space.

2. Design the recursive algorithm

Write a recursive function that takes nodes from both trees. If both are null, return 0. Recursively compute left and right subtree sums from the first tree, set the second node's value to the sum of its original value? Wait, careful: the second tree's node value should be replaced by the sum of the first tree's subtree. So set secondNode.val = firstNode.val + leftSum + rightSum, and return that sum. This modifies the second tree in place.

3. Implement tree helpers

Write a function to build a complete binary tree from a level-order array (using a queue or index arithmetic). Write a function to serialize a tree back to a level-order array (using BFS, omitting trailing nulls).

4. Analyze complexity and edge cases

Explain that the recursion visits each node once, so time is O(n). Space is O(h) due to recursion stack, which for a complete tree is O(log n). Handle edge cases: empty trees, single node, and ensure the second tree is modified correctly.

5. Test and verify

Use the helpers to build sample trees, run the algorithm, and serialize the result to verify correctness. Discuss potential pitfalls like integer overflow and whether to modify in place or create a new tree.

Key Points to Mention

Simultaneous traversal of both trees to compute and assign sums.
Time complexity O(n) because each node is visited once.
Space complexity O(h) due to recursion stack; for complete tree, h = O(log n).
In-place modification of the second tree.
Level-order build and serialize helpers using BFS/queue.
Handling edge cases: empty trees, single node, and ensuring identical structure.

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