BlackRock·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

Jun 2026

Summary

BlackRock software engineer interview with a binary tree traversal question that asked for both recursive and iterative solutions plus complexity analysis. Pretty standard algorithms round but the dual-implementation requirement added a layer I wasn't fully expecting.

Questions Asked (1)

Given the root of a binary tree, implement in-order traversal both recursively and iteratively using a stack, then walk through the time and space complexity of each approach.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

The recursive version came out fine, muscle memory at this point.

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AI HintsAI Generated

Suggested Approach

Start by clearly defining in-order traversal (left, root, right) and then present both recursive and iterative implementations, emphasizing the role of the stack in simulating the call stack. After coding, analyze time and space complexity for each, noting that both are O(n) time and O(h) space, where h is tree height, but the iterative approach uses explicit stack memory while recursion uses implicit call stack.

Pro tip: Mention that the iterative approach can be more memory-efficient for very deep trees because it avoids potential stack overflow, and briefly discuss Morris traversal as an O(1) space alternative to show depth.

1. Clarify traversal order and edge cases

Confirm that in-order traversal visits left subtree, then root, then right subtree. Ask about input constraints (e.g., empty tree, skewed tree) to handle edge cases.

2. Implement recursive solution

Write a simple recursive function that calls itself on left child, processes root, then calls on right child. Explain base case (null node).

3. Implement iterative solution using stack

Use a stack to simulate recursion: push all left children until null, pop and process node, then move to right child. Repeat until stack empty and current node null.

4. Analyze time and space complexity

For both: time O(n) since each node visited once. Space: recursive uses O(h) call stack; iterative uses O(h) explicit stack. Discuss best/worst/average cases (h = log n to n).

5. Compare trade-offs and mention alternatives

Highlight that recursion is simpler but risks stack overflow for deep trees; iterative is more robust but code is longer. Optionally mention Morris traversal for O(1) space.

Key Points to Mention

In-order traversal definition: left, root, right.
Recursive implementation: base case and recursive calls.
Iterative implementation: stack usage and loop condition.
Time complexity: O(n) for both approaches.
Space complexity: O(h) for both, where h is tree height; recursion uses call stack, iterative uses explicit stack.
Trade-offs: recursion simplicity vs. iterative robustness; mention Morris traversal as O(1) space alternative.

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