Given integers n and k such that

0 < n <= 10^6

0 < k <= 10^3

and a string S of length n.

Find any substring of length k in S with maximum number of occurrences. I don't care about the obvious complete search solution. Also, no need for complete solution. just some hints. what topic to look for ?etc.

example

n=13 k=3

abcabcabcabcd

ans is "abc" repeated 4 times.