Databricks·Software Engineer·Technical Phone Screen·Senior

Senior

Apr 2026

Summary

Databricks technical phone screen with a networking/systems-flavored coding problem. The question was more involved than I expected for a phone round, basically a mini firewall simulator with a tricky edge case around CIDR block queries.

Questions Asked (1)

You're given an ordered list of firewall rules, each associating a CIDR range with either 'allow' or 'deny'. First matching rule wins. Given a query that can be either a single IP or a CIDR block, determine whether it's allowed or denied. For CIDR block queries, you need to handle partial overlaps and return 'mixed' if some IPs in the block would be allowed and others denied.

Algorithms & Data StructuresSystem DesignTechnical Trade-offs

Author's notes

The single IP case came together fast.

Create a free account to read the full note

AI HintsAI Generated

Suggested Approach

Clarify the problem constraints and edge cases first, then propose an efficient algorithm using a trie or interval tree to handle first-match semantics and partial overlaps. For CIDR queries, recursively split the query block into smaller sub-blocks until each is entirely covered by a single rule, aggregating results to detect 'mixed'.

Pro tip: Mention that real-world systems often use a trie (e.g., Patricia trie) for longest-prefix matching, but here first-match requires careful ordering; also discuss how to handle rule updates and scalability.

1. Clarify requirements and edge cases

Ask about input size, rule ordering, and whether rules can overlap. Confirm that for CIDR queries, 'mixed' means some IPs allowed and some denied, and that first-match applies to each IP individually.

2. Design data structure for efficient lookup

Propose a trie (prefix tree) where each node represents a bit prefix and stores the first matching rule index. Alternatively, use an interval tree if rules are static. Discuss trade-offs between preprocessing time and query time.

3. Handle single IP queries

Traverse the trie from root to leaf, following the bits of the IP. The first rule encountered along the path determines the action. If no rule matches, default to deny (or as specified).

4. Handle CIDR block queries with partial overlaps

Recursively split the query CIDR into smaller sub-CIDRs until each sub-CIDR is entirely contained within a single rule's range. Aggregate the actions: if all sub-blocks have the same action, return that; otherwise return 'mixed'.

5. Analyze complexity and discuss optimizations

Explain time complexity: O(prefix length) for IP queries, O(number of resulting sub-blocks) for CIDR queries. Mention optimizations like memoization, pruning, or using a segment tree for dynamic updates.

Key Points to Mention

First-match semantics: rules are evaluated in order, so the data structure must preserve priority.
Trie (prefix tree) for efficient IP lookup, with nodes storing the first matching rule.
Handling CIDR queries by recursive splitting and aggregating results to detect 'mixed'.
Edge cases: empty rule list, overlapping rules, query outside all rules, and default action.
Complexity analysis: O(32) for IPv4 IP lookup, O(2^32) worst-case for CIDR but optimized by splitting.
Trade-offs: preprocessing time vs. query time, memory usage, and support for dynamic rule updates.

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