Leetcode 1781. Sum of Beauty of All Substrings

Compute the sum, over all substrings of s (n ≤ 500), of the "beauty" defined as the difference between the most frequent and least frequent characters that appear in the substring. The key challenge is enumerating substrings while maintaining character counts to efficiently compute max/min nonzero frequencies (typical solutions use O(n^2 · 26) counting).