Point72·Machine Learning Engineer·Technical Phone Screen·Intermediate

Intermediate

Apr 2026

Summary

Interviewed for an ML Engineer role at Point72 and got a coding question that looked straightforward but had a subtle wrinkle worth thinking through carefully.

Questions Asked (1)

Given a string of lowercase letters, you must choose exactly one contiguous substring and decrement every character in it by one (with 'a' wrapping around to 'z'). Return the lexicographically smallest result.

Algorithms & Data Structures

Author's notes

My first instinct was to just decrement the whole string, which is obviously wrong because 'a' wraps to 'z' and makes things worse.

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AI HintsAI Generated

Suggested Approach

Recognize that to minimize the string lexicographically, we should decrement the longest prefix of non-'a' characters, stopping at the first 'a' (since decrementing 'a' would make it 'z', which is worse). If the entire string consists of 'a's, decrement the last character to avoid making the string worse. Implement this by scanning the string, applying the decrement to the chosen substring, and returning the result.

Pro tip: Clarify edge cases upfront, especially strings with all 'a's or multiple 'a's, and mention that the operation must be applied exactly once. This shows attention to detail and avoids off-by-one errors.

1. Understand the goal

We need the lexicographically smallest string after decrementing exactly one contiguous substring. Lexicographic order means earlier characters dominate, so we want to make the earliest possible characters smaller.

2. Identify optimal substring

Decrementing a character reduces its value unless it's 'a', which becomes 'z' (worse). So we should decrement a prefix of characters that are not 'a', starting from the first character, and stop at the first 'a'.

3. Handle all-'a' string

If the string consists entirely of 'a's, any decrement will turn an 'a' into 'z', making the string larger. To minimize the damage, decrement only the last character, changing it to 'z'.

4. Implement and test

Scan the string, apply the decrement to the chosen substring, and return the result. Test with cases like 'abc', 'aaa', 'aab', and 'zzz' to verify correctness.

Key Points to Mention

Lexicographic order prioritizes earlier characters, so we aim to reduce the first character if possible.
Decrementing 'a' yields 'z', which is larger, so we avoid decrementing 'a's unless forced.
The optimal substring is the longest prefix without 'a's, starting from index 0.
If the entire string is 'a's, decrement only the last character to minimize the negative impact.
The operation must be applied exactly once, so we cannot skip it even if it makes the string worse.
Time complexity is O(n) with a single pass, and space complexity is O(n) for the output string (or O(1) if modified in place).

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