Instacart·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say

May 2026Remote

Summary

Instacart SWE interview threw a pretty meaty string validation problem at me that I thought I had under control until the follow-up questions started piling on.

Questions Asked (1)

Given a password policy (length 8-20 chars, must have at least one lowercase, one uppercase, one digit, no character repeated 3+ times consecutively), write a function that validates a string and, if invalid, returns the minimum number of insert/delete/replace edits to make it compliant plus one valid corrected password. Walk through your algorithm, handle large inputs, and analyze time and space complexity.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

I started with the validation part, which felt fine, just a regex pass and a length check.

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements and edge cases, then break the problem into two phases: first compute the minimum edits needed to satisfy each constraint independently, then combine them while ensuring the final string meets all constraints. For the corrected password, construct it by applying the edits in a way that resolves multiple violations simultaneously, and discuss how to handle large inputs with a linear-time algorithm.

Pro tip: Mention that the minimum edits can be computed by analyzing the string's length, character composition, and consecutive repeats, and that the corrected password can be built greedily while tracking remaining edits. This shows you can balance correctness with efficiency.

1. Clarify requirements and edge cases

Ask about allowed characters, whether edits can be any character, and if the corrected password must be unique. Confirm that the goal is to minimize total edits and return one valid password.

2. Analyze constraints and compute minimum edits

For each constraint (length, character types, consecutive repeats), compute the minimum edits needed independently. For length, edits = max(0, 8 - len) + max(0, len - 20). For character types, count missing types. For repeats, count the number of groups of 3+ consecutive identical characters and compute edits needed to break them.

3. Combine edits and resolve overlaps

Determine if edits can satisfy multiple constraints at once (e.g., replacing a character can fix both a missing type and a repeat). Use a greedy or dynamic programming approach to find the minimum total edits, ensuring the final length is within bounds.

4. Construct a valid corrected password

Apply the edits to the original string, prioritizing replacements that fix multiple issues. If insertions are needed, add missing character types; if deletions are needed, remove characters from repeat groups. Ensure the final string meets all constraints.

5. Analyze complexity and handle large inputs

The algorithm should run in O(n) time and O(1) extra space (excluding output). For very large inputs, process the string in a single pass, and note that the corrected password can be built on the fly without storing the entire string if only the edit count is needed.

Key Points to Mention

Minimum edits for length: max(0, 8 - len) + max(0, len - 20)
Minimum edits for missing character types: 3 - (number of types present)
Minimum edits for consecutive repeats: sum over groups of floor(group_length / 3)
Overlap resolution: replacements can fix both missing types and repeats, insertions can fix length and missing types, deletions can fix length and repeats
Greedy strategy: prioritize edits that address multiple violations
Time complexity O(n), space complexity O(1) for edit count, O(n) for constructing corrected password

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