Microsoft Software Engineer New Grad interview questions

Apply online... HR phone screen then onsite interviews in Washington. Onsites: Arrive in the morning, have lunch with interviewer then being loop of interviews. All interviews contained whiteboard coding covering DS&Algos, System Design, and Big O analysis.

It's on campus interview. He asked me to write a function to detect whether string1 contains all letters in string2. It's not hard but I think they expect perfect answer. I don't why I was rejected