Perfect Substring : Count the number of substring that contain all digits exactly k times or 0 times.
Eg: If k=2: Valid string is 001122 Invalid String is 00112
Length of string<= 1e5
- Hate Graphs : You are given a list of species numbered 1 to n. You have two arrays a and b. For all i, a[i] and b[i] hate each other. Find the number of ways you can keep these species in enclosures such that no pair of species who hate each other are in the same enclosure.