Longest Consecutive Character Sequence

Watch someone solve the longest consecutive character sequence problem in an interview with a FAANG engineer and see the feedback their interviewer left them. Explore this problem and others in our library of interview replays.

Interview Summary

Problem type
Longest Consecutive Character Sequence
Interview question
Given an unsorted list of lowercase letter characters, where duplicates may appear and order does not matter, find the length of the longest sequence of alphabetically consecutive characters. For example, given the input [D, A, C, B, F], the answer is 4, because A, B, C, D form the longest consecutive sequence. The follow-up asks whether the O(n log n) sorting-based approach can be improved, and the discussion arrives at an O(n) solution using a set-based lookup strategy.

Interview Feedback

Feedback about Pseudo Centaur (the interviewee)

• Advance this person to the next round?
  Yes
• How were their technical skills?
  3/4
• How was their problem solving ability?
  3/4
• What about their communication ability?
  4/4

Overall, you demonstrated good problem-solving ability and were able to identify the right direction for the problem. You also showed good self-awareness during the interview by going back through your code and identifying areas where it could be improved, which is a valuable habit.
Communication & Explanation:
One area to improve is how you explain your code. Try not to explain the implementation line by line. Instead, focus on the core functionality and the reasoning behind the approach.
For example, rather than walking through every line, a stronger explanation would be:
"First, we'll sort the input. Then we'll iterate through the sorted characters and keep track of the current consecutive sequence and the maximum length we've seen."
Code Quality
Be careful with comments that simply repeat what the code already says. For example, a comment immediately next to a line explaining exactly what that line does can create unnecessary noise and may be considered a code smell.
Comments are most useful when they explain why something is being done, rather than simply describing what the code is doing.
Testing
You should also become more comfortable with testing your solutions directly in Python. Python makes it very easy to quickly test different inputs and edge cases.
I’d recommend learning a few basic Python testing techniques and getting into the habit of running examples such as:

• Empty input
• Single element
• Duplicates
• No consecutive characters
• The entire input being consecutive
• Multiple consecutive sequences
  You don’t need a complicated testing framework for an interview. Being comfortable quickly running a few test cases is enough.
  Problem Understanding & Assumptions
  There was also a point where you made an assumption about what "consecutive characters" meant. Be careful with assumptions like this in interviews.
  If something in the problem statement is ambiguous, it's better to explicitly clarify it with the interviewer before building your algorithm around that interpretation.
  A simple question such as:
  "When you say consecutive characters, do you mean alphabetical order, such as a → b → c?"
  can prevent unnecessary work later.
  Optimization
  You ultimately identified the key optimization correctly: we don’t necessarily need to sort the input.
  The important observation is that we need to iterate over the input anyway, so if we can efficiently determine whether a character exists, we can avoid the sorting step and achieve an O(n) average-time solution.
  This is an important pattern to develop:
  Don’t automatically sort just because the input is unordered. First ask whether sorting is actually necessary, or whether another data structure can give you the information you need directly.
  Overall
  You showed good fundamentals and were able to identify the right problem and improve your own solution during the interview.
  The main areas to focus on are more concise technical communication, avoiding unnecessary comments, becoming more comfortable with quick testing in Python, and being more careful about assumptions in ambiguous requirements.
  You also demonstrated a good instinct for optimization by recognizing that the sorting step could be removed. With more practice, the goal should be to identify that kind of optimization earlier and explain the reasoning more clearly and concisely.
  Practice Practice Practice is the key in your situation.

Feedback about Colossal Narwhal (the interviewer)

• Would you want to work with this person?
  Yes
• How excited would you be to work with them?
  4/4
• How good were the questions?
  4/4
• How helpful was your interviewer in guiding you to the solution(s)?
  4/4

Very helpful feedback from the interviewer. I feel he did a great job guiding/correcting if the conversation got stuck without giving away answers. Communicated very well and provided very thorough feedback at the end.