2743. Count Substrings Without Repeating Character

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.

class Solution {
public:
    int numberOfSpecialSubstrings(string s) {
        vector<int> freq(26);
        int res = 0, l = 0, r = 0, n = s.length();
        while(r < n) {
            while(r < n and freq[s[r] - 'a'] == 0) {
                res += (r - l + 1);
                freq[s[r++] - 'a'] += 1;
            }
            while(r < n and freq[s[r] - 'a'] == 1) {
                freq[s[l++]-'a'] -= 1;
            }
        }
        return res;
    }
};