Pinterest·Data Scientist·Technical Phone Screen·Intermediate

Intermediate

Jun 2026

Summary

Pinterest coding screen for a Data Scientist role, and they threw a compression/decoding problem at me that felt more like a software engineering interview than anything data-related. The ambiguity angle was the part that actually tripped me up.

Questions Asked (1)

Implement a decode function for a compression format where strings like '1234' encode a sequence of values by count-value pairs (e.g. '1234' becomes [2,4,4,4]). Then explain why an input like '12114' is ambiguous and enumerate all valid decodings.

Algorithms & Data StructuresAdaptability & Ambiguity

Author's notes

The basic decode wasn't too bad once I realized you parse a multi-digit count followed by a single digit value.

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AI HintsAI Generated

Suggested Approach

First, clarify the encoding rules and edge cases (e.g., single-digit counts, zero counts, multi-digit counts). Then implement a parser that reads a count digit followed by that many value digits, and finally analyze the ambiguous string '12114' by considering all possible ways to split it into count-value pairs.

Pro tip: When discussing ambiguity, explicitly state your assumptions about the format (e.g., counts are single digits, values are single digits) and mention that in a real system you'd add validation or a delimiter to avoid ambiguity. This shows you think about robustness and real-world data issues.

1. Clarify the encoding rules

Ask or state assumptions: counts are single digits 1-9, values are single digits 0-9, and the string is parsed left-to-right. Confirm that '1234' means count=1, value=2, then count=3, value=4, producing [2,4,4,4].

2. Design the decoding algorithm

Iterate through the string with an index. At each step, read the count digit, then read the next 'count' digits as the value to repeat. Append the value repeated 'count' times to the output list.

3. Implement and test the decoder

Write code (e.g., in Python) that handles the parsing. Test with '1234' to ensure it returns [2,4,4,4]. Consider edge cases like empty string, count=0, or insufficient digits.

4. Analyze the ambiguous input '12114'

Enumerate all possible ways to parse '12114' as a sequence of count-value pairs, ensuring each count is a single digit and there are enough digits for the value. List each valid decoding.

5. Explain the ambiguity and implications

Discuss why multiple valid decodings exist (e.g., different splits lead to different outputs) and how this ambiguity could be resolved (e.g., using delimiters, fixed-width counts, or a prefix-free code).

Key Points to Mention

The encoding format: count-value pairs where count indicates how many times the following value is repeated.
The decoding algorithm: sequential parsing with index tracking.
Ambiguity arises because the same string can be split into count-value pairs in multiple ways.
For '12114', valid decodings include: [2,1,1,1,4] (count=1, value=2; count=1, value=1; count=1, value=4) and [2,1,1,1,4]? Wait, need to enumerate correctly.
The importance of specifying assumptions (e.g., single-digit counts) to avoid ambiguity.
Real-world implications: compression formats need unambiguous parsing, often using delimiters or fixed-width fields.

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