828. Count Unique Characters of All Substrings of a Given String

Let's define a function countUniqueChars(s) that returns the number of unique characters in s. - For example, calling countUniqueChars(s) if s = "LEETCODE" then "L", "T", "C", "O", "D" are the unique characters since they appear only once in s, therefore countUniqueChars(s) = 5. Given a string s, return the sum of countUniqueChars(t) where t is a substring of s. The test cases are generated such