Meta Software Engineer Intern interview questions

A professor wants to see if two students have cheated when writing a paper. Design a function : hasCheated(String s1,String s2, int N) that evaluates to true if two strings have a common substring of length N. Additional question after implementation. Assume you don't have the possibility of using String.contains() and String.substring(). How would you implement this. Questions about complexity of my solution. Overall a very friendly interviewer, was saying "Good" the entire time, but no idea if he wanted to keep me focused or if it was really good.