Questions to Ask Before Coding in an Interview: The Senior Engineer's Checklist

The interviewer pastes a problem statement. You read it. You start typing def solution(arr):.

Stop. You have just failed the "Seniority Test."

In the real world, requirements are vague. A "list of numbers" could be 10 items or 10 billion items. It could be sorted or random. It could contain nulls.

If you code without asking, you are guessing. And engineers don't guess.

This guide gives you a tactical checklist of Clarifying Questions to ask in the first 5 minutes of every interview.

TL;DR

• The Goal: Turn an ambiguous prompt into a concrete specification.
• The Mindset: You are not a test-taker; you are a consultant solving a client's problem.
• The Big 3: Ask about Input Constraints, Output Format, and Edge Cases.
• The "Why": These questions determine your algorithm choice (e.g., O(N) vs O(N log N)).
• The Trap: Don't ask generic questions. Ask specific questions that change your code.

Category 1: Input Constraints (The "Scale" Questions)

These questions directly dictate your Time and Space Complexity budget.

1. "What is the range of the input size (N)?"
   • Why: If N < 1000, O(N^2) is fine. If N > 10^5, you need O(N) or O(N log N).
2. "What is the range of the values?"
   • Why: If values are small (e.g., 0-100), you can use Bucket Sort or Counting Sort. If they are negative, it breaks certain sliding window logic.
3. "Can the input be empty or null?"
   • Why: This is the #1 source of runtime errors. Handle it first.
4. "Are the elements unique?"
   • Why: Duplicate values break many binary search and hash map strategies.

Category 2: Data Structure Specifics (The "Shape" Questions)

1. Arrays: "Is the array sorted?" (Unlocks Binary Search/Two Pointers).
2. Graphs: "Is the graph directed or undirected? Is it connected? Can it have cycles?" (Determines traversal safety).
3. Trees: "Is it a Binary Search Tree (BST)? Is it balanced?" (Determines search efficiency).
4. Strings: "Does it contain only lowercase English letters? How do we handle whitespace?"

Category 3: Output & Error Handling (The "Behavior" Questions)

1. "What should I return if no solution exists?"
   • Return -1? Return null? Throw an exception?
2. "Do I need to modify the input in-place, or return a new copy?"
   • Why: In-place saves space but destroys data. A new copy is safer but uses O(N) space. Ask for the preference.
3. "Are there multiple valid answers? If so, which one do you want?"
   • Example: "Return any pair that sums to target" vs "Return the first pair."

Example: Turning Ambiguity into Clarity

Interviewer: "Given a stream of numbers, find the median."

Junior Dev: Starts coding a sorting function.

Senior Dev:

• "Is the stream infinite? Do we need the median at every step or just once?" (Real-time vs Batch).
• "What is the range of numbers? Are they integers?" (Maybe we can use a frequency array).
• "How much memory do I have?" (If it's huge, maybe we need a probabilistic approach).

By asking these, you show you understand System Design implications, not just syntax.

How to Ask Without Being Annoying

Don't just rattle off a list. Weave it into your thought process.

• Bad: "Is it sorted? Are there duplicates? What is N?"
• Good: "I'm thinking about using a Two Pointer approach, which would be O(N). However, that only works if the array is sorted. Is it safe to assume the input is sorted, or should I sort it first?"

This shows Intent. You aren't just stalling; you are gathering data to make an engineering decision.

Tool Tip: Practice the "Setup" Phase

It's hard to remember these questions when you're nervous.

Use LeetCopilot's Interview Mode to practice this specific phase. When the AI presents a problem, spend the first 2 minutes just asking clarifying questions. The AI will respond with constraints, helping you build the habit of "measure twice, cut once."

FAQ

What if the interviewer says "Assume whatever you want"?

This is a test. They want to see what assumptions you make. State the most reasonable assumption and proceed.

• Say: "Okay, I'll assume the input fits in memory and is not sorted. I'll proceed with a Hash Map approach."

Should I write these constraints down?

YES. Write them at the top of your code as comments.

# Constraints:
# 1. N <= 10^5 (Target O(N) or O(N log N))
# 2. Values are positive integers
# 3. Return -1 if no solution

This serves as a contract between you and the interviewer.

Conclusion

The quality of your solution depends on the quality of your understanding.

Clarifying questions are your shield against "Gotcha!" moments. They buy you time, they build rapport, and they prove you are a thoughtful engineer who cares about edge cases.

Never start coding until you know exactly what you are building.