My IP has been banned from luogu for some reason, so I'm asking this question on CF.

Question:

If string S and T with same length has $$$k$$$ or less different characters, we say S matches T,

then if A and B are given, how to find every substring of A that matches B?

My English is poor, so there would be some grammar mistakes, but I think u can understand what I mean.

I don't need trivial solutions such as $$$O(n^2)$$$ (brute) or $$$O(nk\log n)$$$ (hash).