TrialPay Software Engineer interview questions

Met them in career fair and next day they called me for interview. The interview was for 30mins and they asked me to write an algorithm for missing elements in string. I wrote it, but they asked running time of this algorithm. I told n^2 then they ask me to optimize it to n. I did to some extent but dint do it fully. 1 month passed after that. still dint hear back from them. Guess they rejected me.