Count Substrings with Non-Repeating Characters

Given a string, count the number of substrings where all characters are unique (no repeating characters).

Source: LeetCode Discuss - JP Morgan SE3 Interview Experience 2024

A substring is a contiguous sequence of characters within the string. You need to count all substrings where each character appears at most once.

Example: For string "abc", the valid substrings are: "a", "b", "c", "ab", "bc", "abc" = 6 substrings

For string "aba", the valid substrings are: "a", "b", "a", "ab", "ba" = 5 substrings (note: "aba" is not valid because 'a' repeats)