Ositus

#include <algorithm>
#include <iostream>
#include <vector>
using namespace std;

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    string str;
    cin >> str;
    const char *c_str = str.c_str();

    vector<uint64_t> muistio;
    muistio.reserve(str.length());
    muistio[str.length() - 1] = 1;

    for (int64_t i = str.length() - 2; i >= 0; --i) {
        const char *alku = c_str + i;
        uint64_t n = 1;
        uint64_t summa = 0;
        while (*(alku + n) && (find(alku, alku + n - 1, *(alku + n - 1)) == alku + n - 1))
            summa += muistio[i + n++];

        if (find(alku, alku + n - 1, *(alku + n - 1)) == alku + n - 1)
            ++summa;
        muistio[i] = summa % 1000000007;
    }

    cout << muistio[0] % 1000000007 << '\n';
}

Test 1

Group: 1, 2, 3

Verdict: ACCEPTED

Test 2

Group: 1, 2, 3

Verdict: ACCEPTED

Test 3

Group: 1, 2, 3

Verdict: ACCEPTED

Test 4

Group: 1, 2, 3

Verdict: ACCEPTED

Test 5

Group: 2, 3

Verdict: ACCEPTED

Test 6

Group: 3

Verdict: ACCEPTED

Test 7

Group: 3

Verdict: ACCEPTED