Given two strings, find and return the substring of the second string, that's an anagram of the first string.
s1 = "abc"
s2 = "xyzcbaopop"
answer = "cba"
We can try to solve this problem in O(N).
Two strings are anagram, if the they are made of the same characters.
We can observe that any substring of the second string to be an anagram of the first string, must be of the size of the first string.
Thus, we can keep moving a window of size length(s1) and check if it has the same characters as the characters of the first string.