2083. Substrings That Begin and End With the Same Letter

You are given a 0-indexed string s consisting of only lowercase English letters. Return the number of substrings in s that begin and end with the same character.

A substring is a contiguous non-empty sequence of characters within a string.

class Solution {
public:
    long long numberOfSubstrings(string s) {
        vector<int> counter(26,0);
        long long res = 0;
        for(auto& ch : s)
            res += ++counter[ch-'a'];
        return res;
    }
};