Leetcode 2845. Count of Interesting Subarrays

Count the subarrays where the number of elements satisfying nums[i] % modulo == k is itself congruent to k modulo modulo. Reduce to prefix counts of matches taken modulo modulo and count pairs of prefixes where pref[j] ≡ pref[i] + k (mod modulo) — solvable with a running frequency map in linear time.