Design a Compressed String Iterator (Advanced)

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. First, I'd initialize three pointers: one for the current position in the encoded string, one to track the number of remaining instances of the current character, and another to store the current character itself. In the hasNext method, I would iterate through the string to skip over any completed character runs and parse the next digit sequence to update the remaining count and current character. If no more characters exist, it returns false. For the next method, I would first ensure hasNext is true, then return the current character and decrement the remaining count. If the count reaches zero, I advance the main pointer to parse the next digit sequence and character. A critical detail here is handling multi-digit numbers correctly; I must loop until a non-digit character is found to construct the full integer value. For example, given 'a12', the iterator should yield 'a' twelve times before moving on. I would also handle edge cases like trailing digits without characters or empty inputs gracefully. This approach ensures that both next and hasNext operations run in amortized O(1) time relative to the output size, making it highly scalable for large encoded datasets.