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Waymo·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say
Jun 2026

Summary

Waymo coding round with a string decoding problem that's basically a twist on a classic LeetCode problem. Nothing too wild but the nested case tripped me up a bit.

Questions Asked (1)

Q1

Given an encoded string like "abs(cs){3}g", decode it so that substrings inside parentheses are repeated by the number in the following curly braces. Handle nested cases like "a(b(c){2}){2}".

Algorithms & Data Structures
Author's notes

It's a variant of that string decode problem on LeetCode but with different syntax.

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AI HintsAI Generated

Suggested Approach

Use a stack to handle nested parentheses and numbers, processing the string character by character. When encountering a digit, parse the full number; when encountering '(', push the current string and number onto the stack and reset; when encountering ')', pop and repeat the current string by the number, then append to the previous string. This approach naturally handles nesting and multiple repetitions.

Pro tip: Clarify the encoding rules upfront: numbers can be multi-digit, and nested parentheses are allowed. Also, discuss edge cases like empty parentheses or numbers without braces, and consider if the input is guaranteed valid.

1. Understand the problem and clarify rules

Ask clarifying questions about the encoding format: Are numbers always followed by curly braces? Can there be multiple digits? Are there any constraints on nesting depth? Confirm that the output should be the fully decoded string.

2. Choose a data structure and algorithm

Decide to use a stack to manage nested contexts. Each stack element can store the string built so far and the repetition count. Alternatively, use recursion, but be mindful of stack overflow for deep nesting.

3. Iterate through the string and process tokens

Traverse the string character by character. For digits, accumulate the number. For '(', push the current string and number onto the stack, then reset. For ')', pop the previous string and number, repeat the current string, and append to the previous string. For other characters, append to the current string.

4. Handle edge cases and validate

Consider cases like empty input, no parentheses, nested parentheses, multi-digit numbers, and invalid inputs. Test with examples like 'abs(cs){3}g' and 'a(b(c){2}){2}' to ensure correctness.

5. Analyze complexity and optimize if needed

Discuss time and space complexity: O(n) time where n is the length of the decoded string (or input length if we consider output size), and O(d) space for the stack where d is the nesting depth. Mention potential optimizations like using a StringBuilder for efficiency.

Key Points to Mention

  • Stack-based approach for handling nested structures
  • Parsing multi-digit numbers correctly
  • Time and space complexity analysis
  • Edge cases: empty parentheses, no repetition, deep nesting
  • Alternative recursive solution and its trade-offs
  • Use of StringBuilder for efficient string concatenation

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