Snowflake·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

May 2026

Summary

Snowflake SWE interview with a tree traversal problem that looked straightforward on the surface but had enough nuance to trip you up if you weren't careful about boundary conditions.

Questions Asked (1)

Given a full, complete, and balanced binary tree, return all boundary nodes in counter-clockwise order: root first, then the left boundary top-down, then leaves left to right, then the right boundary bottom-up. No duplicates.

Algorithms & Data Structures

Author's notes

Variant of a known LC problem but the full+complete+balanced constraint changes things a bit.

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AI HintsAI Generated

Suggested Approach

Break the problem into four distinct traversals: root, left boundary (excluding leaves), leaves (left to right), and right boundary (excluding leaves, bottom-up). Use DFS to collect nodes while carefully avoiding duplicates by excluding leaves from boundary traversals and handling edge cases like single-node trees.

Pro tip: Clarify whether the tree is guaranteed to be complete and balanced, as this affects edge cases. Also, explicitly state that you'll exclude leaves from left/right boundary to avoid duplicates, showing attention to detail.

1. Clarify and define boundaries

Confirm the definition of boundary nodes: root, left boundary (excluding leaves), leaves, right boundary (excluding leaves). Discuss edge cases like empty tree, single node, or skewed tree.

2. Collect root and left boundary

Add root if it exists. Traverse left boundary top-down: at each node, if it's not a leaf, add it, then move to left child if exists, else right child.

3. Collect leaves left to right

Perform a DFS (preorder) to collect all leaf nodes in left-to-right order. Ensure leaves are added only once and not duplicated with boundary nodes.

4. Collect right boundary bottom-up

Traverse right boundary top-down but store nodes in a temporary list, then reverse it before adding to result. Exclude leaves and avoid duplicates.

5. Combine and return

Concatenate the four parts in order: root, left boundary, leaves, right boundary (reversed). Return the final list.

Key Points to Mention

Handling edge cases: empty tree, single node, tree with only left or right children.
Avoiding duplicates by excluding leaves from left and right boundary traversals.
Time complexity: O(n) where n is number of nodes, as each node is visited at most twice.
Space complexity: O(h) for recursion stack, where h is height of tree.
Using iterative or recursive DFS for boundary and leaf collection.
Ensuring leaves are collected in left-to-right order using preorder traversal.

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