Interview Day Guide

Practical, printable reference for technical interview day.

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What to Print & Bring

Printed Materials (in this order on your desk):

1. 07-interview-day-guide.md (this sheet) -- top of stack for quick pattern lookup
2. 01-python-stdlib.md -- collections, heapq, itertools, bisect
3. 03-algorithm-templates.md -- sliding window, binary search, BFS/DFS, DP, backtracking
4. 05-common-patterns.md -- pattern recognition triggers
5. 04-big-o-complexity.md -- complexity verification
6. 06-system-design.md -- for system design round (L7+)
7. RESEARCH.md first page, if available (organization overview, mission, products, tech stack)

Physical Items:

• Notepad + pen (for sketching graphs, trees, state transitions)
• Water bottle
• Charger plugged in

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Pre-Interview Warmup (30 min before)

1. Solve 1 easy problem (10 min) -- src/algo/arrays/two_sum.py or src/algo/stacks_queues/valid_parentheses.py. Gets your brain in coding mode.
2. Quick review (10 min) -- skim reference sheet 01 (Python stdlib) + 03 (algorithm templates). Focus on function signatures, not reading every line.
3. Body prep (10 min) -- stretch, hydrate, 3 deep breaths. Use the bathroom. Close Slack/email. Set phone to DND.

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Browser Tabs to Open

Open these before the interview starts:

Tab	URL	Purpose
1	<https://docs.python.org/3/library/collections.html>	Counter, defaultdict, deque
2	<https://docs.python.org/3/library/heapq.html>	heappush, heappop, nlargest, nsmallest
3	<https://docs.python.org/3/library/itertools.html>	combinations, permutations, accumulate
4	<https://docs.python.org/3/library/bisect.html>	bisect_left, bisect_right, insort
5	<https://www.bigocheatsheet.com/>	Data structure & sorting complexities
6	<https://visualgo.net/>	Algorithm visualization (graphs, sorting, trees)
7	<https://docs.python.org/3/library/typing.html>	Type hints reference
8	Target organization/product pages	Mission, product, and leadership context

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Interview Flow & Prep by Round

Stage 1: Pre-screen

Headhunter chats with CTO and hiring manager based on FOSS contributions, PR experience, etc. No prep needed -- this is about your existing body of work.

Stage 2: Hiring manager interview

Role fit, experience, expectations. Be ready to discuss your background, why the organization, and what you bring.

Stage 3: Engineering lead interview

Technical depth, architecture thinking, team dynamics. Print sheet on desk: 06 (system design) for reference.

Stage 4: Backend algo deep dive (1-3 hrs)

Tabs: 1-5 for quick stdlib lookups and complexity verification. Print sheets on desk: 01, 03, 04, 05.

Approach for each problem:

1. Read the problem fully -- twice. Underline constraints.
2. State your approach out loud before coding.
3. Write clean code with type hints.
4. Test with the given examples + one edge case.
5. State time and space complexity.

Think out loud the entire time. Silence = lost signal.

Stage 5: Frontend / cross-team engineering deep dive (1-3 hrs)

Tabs: None needed (they show you code). Print sheet on desk: 01 for stdlib reference, 05 for common patterns.

5-step framework for code review:

1. Read slowly -- skim signatures and docstrings first, then trace one path.
2. Identify the "smell" -- nested loops, missing error handling, N+1 queries, unbounded cache.
3. Trace with concrete input -- pick a small input and walk through the code line by line.
4. Name the pattern/antipattern -- "This is an N+1 query pattern" or "This cache has no eviction policy."
5. Propose fix with complexity analysis -- "Replace the nested loop with a hash map lookup, O(n^2) -> O(n)."

Problem decomposition framework:

1. Restate the problem in your own words. Confirm understanding.
2. Identify sub-problems. Draw a diagram (use your notepad).
3. Map each sub-problem to a known algorithm or data structure.
4. Define interfaces between components.
5. Discuss tradeoffs: time vs space, accuracy vs speed, complexity vs maintainability.

Stage 6: On-site intensive (6-8 hrs)

All-day in-person. Multiple rounds covering algorithms, system design, code reading, and collaboration.

System design framework (30 min per round):

• 5 min: Requirements -- functional + non-functional. Ask clarifying questions. Estimate scale (QPS, data volume, latency target).
• 5 min: High-level design -- draw boxes and arrows. Identify core components.
• 15 min: Deep-dive components -- pick 2-3 to detail. Schema, APIs, data flow, failure modes.
• 5 min: Tradeoffs -- consistency vs availability, cost vs performance. Name alternatives you considered.

Stage 7: Executive or bar-raiser interview

Tab: 8 (target organization/product pages).

Prepare 3-4 questions showing genuine interest:

• "How is the mission evolving across operational domains?"
• "What's the biggest technical challenge the engineering team is tackling right now?"
• "How does the team balance mission needs, reliability, and product velocity?"
• "Which constraints shape engineering decisions most: latency, security, scale, certification, or operator workflow?"

Mission, judgment, team fit. Be authentic, curious, and concise.

Stage 8: Offer & negotiation

No prep in this guide -- but know your numbers and priorities going in.

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Quick-Access Patterns Checklist

When you see a trigger word in the problem statement, reach for the corresponding pattern:

Trigger	Pattern	Template Location
"subarray", "substring", "contiguous"	Sliding window	ref 03 SS3
"sorted array", "search", "minimize maximum"	Binary search	ref 03 SS1
"shortest path", "level by level", "minimum steps"	BFS	ref 03 SS2
"connected components", "group", "union"	DFS / Union-Find	ref 03 SS2, SS6
"k-th largest/smallest", "top k", "closest k"	Heap	ref 02 SS(heap)
"pairs that sum to", "two values"	Two pointers / hash map	ref 05 SS2
"overlapping intervals", "merge ranges"	Merge intervals / sort by start	ref 05 SS(intervals)
"all combinations", "all subsets", "generate"	Backtracking	ref 03 SS5
"maximum/minimum cost", "number of ways"	Dynamic programming	ref 03 SS4
"dependency order", "prerequisites", "schedule"	Topological sort	ref 03 SS(topo)
"prefix", "autocomplete", "word search"	Trie	ref 02 SS(trie)
"next greater", "previous smaller", "monotonic"	Monotonic stack	ref 05 SS(stack)
"sliding window maximum/minimum"	Monotonic deque	ref 05 SS(deque)
"cycle detection", "linked list middle"	Fast & slow pointers	ref 05 SS(pointers)
"palindrome", "parentheses matching"	Stack	ref 02 SS(stack)

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Emergency Debug Checklist

When you're stuck during a coding round:

Symptom	Check
Off-by-one error	Check loop bounds: < n vs <= n. Check index ranges: 0-indexed vs 1-indexed.
Wrong answer	Trace with the simplest possible input (n=1 or n=2).
Time limit exceeded (TLE)	Look for O(n^2) that can become O(n log n) or O(n). Common fix: replace inner loop with hash map or binary search.
Can't think of approach	Ask: "What data structure gives me O(1) lookup?" (hash map). "Can I sort first?" "Can I use a seen set?"
Still stuck	Simplify: solve for a smaller version first. Build from brute force, then optimize.
Recursive solution has bugs	Check base cases first. Draw the recursion tree for n=3.
Memory limit exceeded	Are you storing all results? Can you stream / use constant space? Iterative DP instead of memoization?

Golden rule: If stuck for more than 3 minutes, talk to your interviewer. Say: "I'm considering X and Y approaches. X has this tradeoff, Y has that. I'm leaning toward X because..." They want to help.