I just received a phone call from a recruiter and then had a phone interview. I needed to write code on Google Docs, but the problem was too difficult for me. The interviewer was very nice and provided many hints to help me finish the major part of the code. I think I spoke too little during the whole process, which was the major reason I failed.

Given a string, find the longest substring T that contains m distinct characters. (Need O(n) solution)