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Databricks·Software Engineer·Technical Phone Screen·Senior

Senior
May 2026

Summary

Databricks SWE interview with a pretty gnarly networking/algorithms problem. The kind of question that sounds manageable until you're actually implementing CIDR subtraction on a whiteboard.

Questions Asked (1)

Q1

Given a target CIDR block and an ordered list of allow/deny CIDR rules, determine whether the target is fully covered by allow rules. Processing must stop immediately on any deny overlap, and allow rules should subtract their overlapping portion from the target. Return true only if the target ends up completely covered.

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

This one required actually implementing CIDR intersection and then subtraction, which can split a block into multiple smaller CIDRs.

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AI HintsAI Generated

Suggested Approach

Model CIDR blocks as integer intervals and process rules in order, maintaining a set of uncovered sub-intervals of the target. For each allow rule, subtract its overlap from the uncovered set; for each deny rule, if it overlaps any uncovered part, immediately return false. At the end, return true only if the uncovered set is empty.

Pro tip: Explicitly state that you're treating CIDRs as integer intervals to simplify overlap and subtraction, and mention that you'd handle edge cases like /32 and /0. This shows you understand the underlying representation and can avoid common pitfalls.

1. Clarify and define representation

Confirm that rules are processed in order and that deny rules cause immediate failure on any overlap with the target. Represent each CIDR as an integer interval [start, end] using bitwise operations.

2. Initialize uncovered set

Start with the target CIDR as a single uncovered interval. This set will track portions of the target not yet covered by allow rules.

3. Process rules sequentially

For each rule: if deny, check if it overlaps any uncovered interval; if so, return false immediately. If allow, subtract its overlap from each uncovered interval, splitting intervals as needed.

4. Check final coverage

After processing all rules, return true if the uncovered set is empty, meaning the target is fully covered by allow rules; otherwise return false.

5. Analyze complexity and edge cases

Discuss time complexity (O(n * m) where n is number of rules and m is number of intervals) and edge cases like /0, /32, adjacent intervals, and rules outside the target.

Key Points to Mention

  • CIDR to integer interval conversion using bitwise operations (e.g., start = ip & mask, end = start | ~mask).
  • Interval subtraction algorithm: splitting an interval when an allow rule partially overlaps.
  • Immediate termination on deny overlap: checking intersection with any uncovered interval.
  • Data structure choice: maintaining a list of disjoint uncovered intervals, possibly sorted for efficiency.
  • Edge cases: /0 (entire IPv4 space), /32 (single IP), rules that don't overlap the target, and adjacent intervals that could be merged.
  • Time and space complexity analysis, and potential optimizations like merging intervals or using a segment tree.

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