0828. Count Unique Characters of All Substrings of a Given String

https://leetcode.com/problems/count-unique-characters-of-all-substrings-of-a-given-string

Description

Let's define a function countUniqueChars(s) that returns the number of unique characters on s.

• For example 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.

Notice that some substrings can be repeated so in this case you have to count the repeated ones too.

Example 1:

**Input:** s = "ABC"
**Output:** 10
**Explanation:** All possible substrings are: "A","B","C","AB","BC" and "ABC".
Evey substring is composed with only unique letters.
Sum of lengths of all substring is 1 + 1 + 1 + 2 + 2 + 3 = 10

Example 2:

**Input:** s = "ABA"
**Output:** 8
**Explanation:** The same as example 1, except countUniqueChars("ABA") = 1.

Example 3:

**Input:** s = "LEETCODE"
**Output:** 92

Constraints:

• 1 <= s.length <= 105

• s consists of uppercase English letters only.

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