Count_Number_of_Homogenous_Substrings.cpp

class Solution {
public:
    static int findLastDup(const string& s, int left_index) {
        auto start = s.begin() + left_index;
        auto end = s.end();

        // Find the first character that is not equal to *start.
        auto non_match = std::find_if_not(start, end, [start](char c) {
            return c == *start;
        });

        // Calculate the distance from start to the first non-matching character.
        return std::distance(start, non_match);
    }

    int countHomogenous(string s) {
        long long total_substrings = 0;

        for (int i = 0; i < s.length(); ) {
            int length = findLastDup(s, i);
            // Use the arithmetic series sum formula: n * (n + 1) / 2
            total_substrings += ((long long)length * (length + 1)) / 2;
            i += length; // Adjust i to skip the duplicate characters.
        }
        return (total_substrings % 1000000007);
    }
};