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Databricks·Software Engineer·Technical Phone Screen·Senior

Senior
May 2026

Summary

Databricks software engineering interview with a pretty gnarly networking/CIDR problem. The question had enough moving parts that I spent a good chunk of time just making sure I understood the semantics before writing a single line.

Questions Asked (1)

Q1

Given a target IPv4 CIDR block and an ordered list of allow/deny rules (each with their own CIDR), implement a function that returns true if the target block is fully covered by allow rules without any deny rule overlapping the remaining uncovered region. Rules must be processed sequentially, and allow rules subtract their overlap from what remains; a single deny overlap with remaining IPs returns false immediately.

Algorithms & Data StructuresTechnical Trade-offsSystem Design
Author's notes

This one took me a while to even parse correctly.

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AI HintsAI Generated

Suggested Approach

Clarify the problem by restating the sequential rule processing and the coverage condition. Then, describe an algorithm that represents the target CIDR as a set of IP ranges, processes each rule in order, and maintains the remaining uncovered ranges. For each allow rule, subtract its overlap from the remaining ranges; for each deny rule, if it overlaps any remaining range, return false immediately. Finally, return true if no remaining ranges are left after processing all rules.

Pro tip: Mention that CIDR blocks can be converted to integer ranges for efficient overlap and subtraction operations, and that using a sorted list of disjoint ranges keeps the algorithm clean and O(n log n) or better. Also, discuss edge cases like rules that are subsets or supersets of the target, and the importance of handling IPv4 integer conversion correctly.

1. Clarify requirements and constraints

Restate the problem to ensure understanding: rules are processed in order, allow rules subtract from remaining uncovered IPs, and any deny rule overlapping remaining IPs causes immediate false. Ask about input format, rule ordering, and whether the target CIDR is guaranteed to be valid.

2. Choose data representation

Convert CIDR blocks to integer ranges [start, end] for easy overlap and subtraction. Represent the remaining uncovered IPs as a list of disjoint ranges, initially containing the target range.

3. Design algorithm for rule processing

Iterate through rules sequentially. For an allow rule, subtract its range from each remaining range, updating the list. For a deny rule, check if it overlaps any remaining range; if so, return false immediately.

4. Implement range subtraction and overlap check

Write helper functions to subtract a range from a list of disjoint ranges and to check if a range overlaps any in the list. Ensure subtraction handles partial overlaps and splits ranges correctly.

5. Analyze complexity and edge cases

Discuss time complexity: O(n * m) where n is number of rules and m is number of remaining ranges, but can be optimized. Cover edge cases: empty rules, rules outside target, allow rules that fully cover, deny rules that don't overlap, and IPv4 integer conversion boundaries.

Key Points to Mention

  • Sequential processing: order matters, so rules cannot be reordered or combined arbitrarily.
  • CIDR to integer range conversion: essential for efficient overlap and subtraction operations.
  • Remaining uncovered ranges: maintain a list of disjoint ranges to track what's left.
  • Immediate false on deny overlap: any deny rule that intersects remaining IPs invalidates the coverage.
  • Complexity analysis: worst-case O(n * m) but can be optimized with interval trees or sorting.
  • Edge cases: rules that are subsets/supersets, empty rules, and boundary conditions like /0 and /32.

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