Given a string s, return its total appeal, which is calculated by summing the appeals of all its substringsA substring is a contiguous sequence of characters within a string..

The appeal of a string is defined as the count of unique characters present in that string.

For instance, the appeal of “xyzxz” is $3$, as it contains three distinct characters: ‘x’, ‘y’, and ‘z’.

Constraints:

• $1 \leq$ s.length $\leq 10^3$

• s consists of only lowercase English letters.

The key intuition for solving this problem efficiently is to focus on the contribution of each character to the total appeal, rather than processing all possible substrings. In any substring, each character contributes only once to the total appeal, regardless of how many times it appears. Therefore, we can consider only the first occurrence of each character in any substring as contributing to the total appeal. To achieve this, we keep track of the last index of each character, ...