The beauty of a string is the difference in frequencies between the most frequent and least frequent characters. For example, the beauty of "abaacc" is 3 - 1 = 2. Given a string s, return the sum of beauty of all of its substrings. Example 1: Input: s = "aabcb" Output: 5 Explanation: The substrings with non-zero beauty are ["aab","aabc","aabcb","abcb","bcb"], each with beauty equal to 1. Example 2: Input: s = "aabcbaa" Output: 17 Constraints: 1 <= s.length <= 500 s consists of only lowercase English letters.