Design a Compressed String Iterator (Advanced)
Design an iterator for a run-length encoded string that handles sequences of characters and numbers, supporting `next` and `hasNext` efficiently.
Why Interviewers Ask This
Stripe evaluates this question to assess a candidate's ability to optimize space complexity while maintaining efficient time complexity in iterator design. They specifically look for understanding of run-length encoding edge cases, such as multi-digit numbers and empty strings, and the capacity to implement stateful logic without reconstructing the entire string in memory.
How to Answer This Question
1. Clarify requirements: Confirm if input contains only digits followed by characters or mixed formats, and define behavior for invalid inputs.
2. Analyze constraints: Discuss how storing the full decoded string violates O(1) space goals, necessitating a pointer-based approach.
3. Define state variables: Propose tracking current character, remaining count, and an index pointer into the compressed string.
4. Draft hasNext logic: Explain how to skip past digit sequences to find the next available character efficiently.
5. Implement next method: Detail parsing multi-digit counts, decrementing the counter, and returning the character while updating internal state.
6. Edge case review: Test scenarios like single-character runs, end-of-string boundary conditions, and consecutive digits.
Key Points to Cover
- Explicitly stating the decision to use O(1) space instead of pre-decoding
- Demonstrating correct parsing logic for multi-digit integers within the string
- Separating hasNext and next logic to prevent redundant traversals
- Handling boundary conditions where the string ends mid-sequence
- Discussing time complexity trade-offs between initialization and iteration
Sample Answer
To solve the Compressed String Iterator problem efficiently, I would avoid decoding the entire string upfront to maintain O(1) space complexity, which aligns with Stripe's focus on performance and resource efficiency. Fi…
Common Mistakes to Avoid
- Decoding the entire string immediately, which fails the space complexity constraint
- Failing to parse multi-digit numbers correctly, treating each digit as a separate count
- Not updating the internal state pointer after a character is fully exhausted
- Ignoring edge cases where the string ends with a number but no following character
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