Repeatedly Add Consecutive Equal Digits

Problem Overview

• Given a string of digits, repeatedly replace each run of consecutive equal digits with the sum of that run until no adjacent digits are equal; return the final string.
• Input: digit-only string. Output: transformed digit string. Sums can be multi-digit; iterate until stability (no repeats).
• Domain: string processing and iterative aggregation in numeric text data.
• From Uber interviews; a coding interview problem often used as an interview question.