C3 AI·Data Scientist·Technical Phone Screen·Intermediate

Intermediate

Jun 2026

Summary

Interviewed for a Data Scientist role at C3 AI and got a coding problem that looked deceptively simple at first glance. It was a technical screen with one meaty algorithmic question about string decoding.

Questions Asked (1)

Given an encoded string where a number followed by a bracketed substring means the substring repeats that many times (and encodings can be nested), write a function to decode and return the full decompressed string.

Algorithms & Data Structures

Author's notes

I knew immediately it was a stack problem but spent a weirdly long time second-guessing myself on the nested case.

Create a free account to read the full note

AI HintsAI Generated

Suggested Approach

Use a stack-based approach to handle nested encodings: iterate through the string, pushing characters and numbers onto a stack until a closing bracket is encountered, then pop and repeat the substring accordingly. Alternatively, use recursion to decode each bracketed section. Clearly explain the algorithm, then implement it efficiently, ensuring correct handling of multi-digit numbers and nested patterns.

Pro tip: Before coding, clarify edge cases such as empty strings, multi-digit repeat counts, and deeply nested encodings; this shows thoroughness and prevents bugs. Also, mention that the solution runs in O(n) time where n is the length of the decoded string, which is optimal.

1. Understand the problem and clarify edge cases

Restate the problem in your own words and ask clarifying questions about input format, constraints, and edge cases (e.g., empty string, multi-digit numbers, nested brackets).

2. Choose an approach: stack or recursion

Decide between using a stack (iterative) or recursion. Explain the trade-offs: stack avoids recursion depth limits, recursion is more intuitive for nested structures.

3. Outline the algorithm step-by-step

Describe how to parse the string: maintain a current string and repeat count; on '[', push current state and reset; on ']', pop and repeat; on digits, build the number; on letters, append to current string.

4. Implement the solution in code

Write clean, modular code with meaningful variable names. Handle multi-digit numbers and nested brackets correctly. Test with a few examples.

5. Analyze complexity and test edge cases

State time and space complexity (O(n) time, O(n) space). Walk through edge cases like '3[a]2[bc]', '3[a2[c]]', and '2[abc]3[cd]ef' to verify correctness.

Key Points to Mention

Use a stack to store previous strings and repeat counts for nested encodings.
Parse multi-digit numbers by accumulating digits until a non-digit is encountered.
On encountering '[', push the current string and current number onto the stack, then reset them.
On encountering ']', pop the previous string and number, repeat the current string, and append to the previous string.
Time complexity is O(n) where n is the length of the decoded string; space complexity is O(n) for the stack.
Edge cases: empty string, no brackets, nested brackets, multi-digit repeat counts, and characters outside brackets.

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