Apple·Machine Learning Engineer·Technical Phone Screen·Senior

SeniorPrefer not to say

Jun 2026

Summary

Apple MLE screen with a coding problem that looks deceptively simple until you realize they want you to throw out the DP solution and think linearly. Pretty focused session, just the one algorithmic problem plus a follow-up discussion.

Questions Asked (1)

Given two strings, determine whether they are exactly one edit distance apart, where an edit is an insertion, deletion, or substitution of a single character.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

My first instinct was full edit-distance DP and I started going down that road before they steered me away from it.

Create a free account to read the full note

AI HintsAI Generated

Suggested Approach

Start by clarifying the problem and edge cases, then propose a linear time solution that handles the three edit types by comparing string lengths and scanning for the first difference. Walk through the algorithm step-by-step, and analyze time and space complexity.

Pro tip: Mention that this problem is a simplified version of edit distance and that your solution runs in O(n) time and O(1) space, which is optimal. Also, relate it to real-world applications like spell checkers or DNA sequence alignment to show practical insight.

1. Clarify requirements and edge cases

Ask if the strings can be empty, if they are case-sensitive, and confirm that exactly one edit means the strings are not equal but differ by one operation. Discuss edge cases like length difference greater than 1.

2. Outline the algorithm

If length difference > 1, return false. If lengths are equal, check for exactly one substitution by scanning and counting mismatches. If lengths differ by 1, check for exactly one insertion/deletion by scanning with two pointers.

3. Walk through an example

Choose a representative example (e.g., 'cat' and 'cut' for substitution, 'cat' and 'cats' for insertion) and trace the algorithm to demonstrate correctness.

4. Analyze complexity

State that the time complexity is O(n) where n is the length of the shorter string, and space complexity is O(1) since only a few variables are used.

5. Discuss trade-offs and extensions

Mention that this approach is optimal for the given problem. For multiple edits, a dynamic programming approach would be needed, but that would be overkill here.

Key Points to Mention

Length difference check: if absolute difference > 1, return false immediately.
Two-pointer technique for insertion/deletion case: when a mismatch is found, skip one character in the longer string and continue.
Substitution case: count mismatches; if more than one, return false.
Early termination: return false as soon as a second edit is detected.
Time and space complexity: O(n) time, O(1) space.
Real-world relevance: spell checkers, DNA sequence alignment, and error correction in ML data preprocessing.

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