NVIDIA·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

Jun 2026

Summary

NVIDIA coding screen with one problem that looks manageable until you actually sit down and implement it. The encoding rules stack in ways that trip you up if you're not careful about parsing order.

Questions Asked (1)

You're given an encoded string that maps to letters a–z using three rules: a single digit 1–9 maps to the corresponding letter by position, a number followed by '#' maps to letters j–z (so '10#' is j, '26#' is z), and a parenthesized number after a token means repeat that letter that many times. Decode the full string and return a frequency array of length 26.

Algorithms & Data Structures

Author's notes

I got the basic digit-to-letter mapping fine, but the '#' token tripped me up because you have to look ahead (or behind, depending on direction) to figure out if you're dealing with a one- or two-digit number.

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AI HintsAI Generated

Suggested Approach

Parse the string left-to-right using a state machine that handles three token types: single digits, digits followed by '#', and optional repetition suffixes in parentheses. For each token, determine the letter and its repeat count, then increment the frequency array accordingly. Validate edge cases like multi-digit repeats and ensure the final array sums to the total decoded length.

Pro tip: Clarify with the interviewer whether the input is guaranteed valid and whether repeat counts can be multi-digit or zero; this shows attention to detail and avoids incorrect assumptions. Also, consider memory efficiency by updating the frequency array directly instead of building the decoded string.

1. Clarify rules and edge cases

Confirm the exact mapping rules, especially how numbers followed by '#' work (e.g., '10#' to '26#'), and whether repeat counts can be multi-digit or zero. Ask about input validity and constraints.

2. Design parsing strategy

Plan a single-pass parser that reads characters and identifies tokens: a digit not followed by '#' or '(' is a single-digit letter; a number followed by '#' is a two-digit letter; a parenthesized number after a token indicates repetition.

3. Implement tokenization and repetition

Iterate through the string, extract the letter and its repeat count for each token, and update the frequency array by adding the repeat count to the appropriate index.

4. Handle edge cases and validate

Test with cases like '1(2)' (should give two 'a's), '10#(3)' (three 'j's), and multi-digit repeats like '1(12)'. Ensure the frequency array sums to the total decoded length.

5. Analyze complexity and optimize

State that the solution is O(n) time and O(1) space (since the frequency array is fixed size). Discuss potential optimizations like avoiding string concatenation.

Key Points to Mention

Tokenization: distinguishing between single-digit letters, '#'-suffixed letters, and repetition suffixes.
Mapping logic: converting numeric values to letters (1-9 to a-i, 10#-26# to j-z).
Repetition handling: parsing parenthesized numbers and applying the repeat count.
Frequency array update: incrementing the correct index by the repeat count.
Edge cases: multi-digit repeat counts, zero repeats, and invalid inputs.
Complexity: O(n) time and O(1) space, with n being the length of the encoded string.

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