< Ace Coding /> 🚀
/*
You are given a string s consisting only of lowercase English letters.
We call a substring special if it contains no character which has occurred at least twice (in other words, it does not contain a repeating character).
Your task is to count the number of special substrings.
For example, in the string "pop", the substring "po" is a special substring, however, "pop" is not special (since 'p' has occurred twice).
Return the number of special substrings.
A substring is a contiguous sequence of characters within a string. For example, "abc" is a substring of "abcd", but "acd" is not.
Example 1:
Input: s = "abcd"
Output: 10
Explanation: Since each character occurs once, every substring is a special substring.
We have 4 substrings of length one, 3 of length two, 2 of length three, and 1 substring of length four. So overall there are 4 + 3 + 2 + 1 = 10 special substrings.
Example 2:
Input: s = "ooo"
Output: 3
Explanation: Any substring with a length of at least two contains a repeating character. So we have to count the number of substrings of length one, which is 3.
Example 3:
Input: s = "abab"
Output: 7
Explanation: Special substrings are as follows (sorted by their start positions):
Special substrings of length 1: "a", "b", "a", "b"
Special substrings of length 2: "ab", "ba", "ab"
And it can be shown that there are no special substrings with a length of at least three. So the answer would be 4 + 3 = 7.
l r
a b c d a
Constraints:
1 <= s.length <= 10^5
s consists of lowercase English letters
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