Design a Compressed String Iterator
Design an iterator that supports the `next` and `hasNext` operations for a run-length encoded string (e.g., 'L1e2t1c1o1d1e1').
Why Interviewers Ask This
Stripe asks this to evaluate a candidate's ability to handle stateful iteration over compressed data efficiently without loading the entire string into memory. It tests understanding of pointers, loop invariants, and edge cases like multi-digit counts or empty inputs. The problem specifically probes whether you can design an O(1) amortized solution that balances time complexity with space constraints, a critical skill for high-throughput payment systems.
How to Answer This Question
Start by clarifying requirements: confirm if counts are always single digits or if they can be multi-digit integers. Propose a two-pointer approach where one pointer tracks the current character and another tracks the remaining count. Step 1: Parse the input string once during initialization to build a list of (char, count) pairs or maintain indices. Step 2: Implement hasNext by checking if the current index is within bounds and the current count is greater than zero. Step 3: For next, decrement the current count; if it hits zero, advance the character pointer to the next pair. Step 4: Handle edge cases immediately, such as empty strings or invalid formats. Step 5: Discuss time and space complexity, emphasizing that while parsing takes O(N), each next/hasNext call is O(1). This structured breakdown shows systematic thinking valued at Stripe.
Key Points to Cover
- Demonstrate O(1) time complexity for next and hasNext operations
- Show clear handling of multi-digit numbers in the run-length encoding
- Explain how you manage state without storing the full decompressed string
- Address edge cases like empty strings or zero counts explicitly
- Articulate the trade-off between initial parsing cost and runtime efficiency
Sample Answer
To solve the Compressed String Iterator, I first clarify that the input format uses run-length encoding where numbers can be multi-digit. My approach involves maintaining a pointer to the current character and a counter…
Common Mistakes to Avoid
- Decompressing the entire string upfront, which violates space constraints
- Failing to handle multi-digit numbers, assuming all counts are single digits
- Not updating the character pointer correctly when the count reaches zero
- Ignoring edge cases like an empty input string or malformed sequences
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