Why is this problem categorized as EASY?
Unless everybody is using the naive algorithm, or even worse--string library function.
Maybe because of the test cases. If you implement a naive O(nm) greedy algorithm you get accepted. Note that the keyword is greedy.
I guess it's because we usually aren't expected to implement KMP in a real interview. So this question probably will be treated as a warmup question and brute force is enough.
But anyway, you can find all good answers in here: