Amazon's São Paulo interview process includes an online application, logic, and programming tests.

This is followed by on-site interviews covering technical, behavioral, and cultural aspects, with a focus on alignment with their Leadership Principles.

Adequate preparation is crucial.

Given a string containing alphanumeric characters, write a function to find the longest substring with at most K distinct characters. If multiple substrings of the same length exist, return the first one found. Optimize the solution for both time and space complexity.