Ailoitte Flutter Developer interview questions

i applied for flutter developer. first they give me a assignment, i completed that then they scheduled a technical interview. in the interview they asked several basic questions, i answered all questions correctly. after that they give me some question to solve on dart pad. first question reverse the string, i successfully run that. then next question to remove the duplicate from a list i also do that, still they reject me. i don't know what they need, i ask him no one answer the reason. why these people waste our times and efforts i don't know