Find Smallest Appealing

Problem Overview

• Find the smallest attractive product code new_code such that new_code ≥ old_code.
• A code is attractive if its digits repeat every k positions (b[i] = b[i+k]).
• Input: decimal string old_code of length n and integer k (1 ≤ k < n). Output: decimal string new_code with no leading zeros.
• Context: generating a more appealing Amazon product code for an online shopping event.
• From Amazon interviews; a coding interview problem and common interview question.