Ositus

#include <iostream>
#include <string>
#include <math.h>
using namespace std;

long all(long len) {
    return pow(2, max(len - 1, (long) 0));
}

long solve(string value, bool debug = false, bool fixUnique = false) {
    long res = 1;

    for (int i = 1; (unsigned) i < value.length(); i++) {
        bool isDup = false;
        long pos = -1;
        for (int j = i; j >= 0; j--) {
            if (value[i] == value[j] && i != j) {
                fixUnique = false;
                isDup = true;
                pos = j;
                break;
            }
        }

        if (isDup) {
            long start = solve(value.substr(0, pos + 1), false, false); // /*/ all(pos + 1) /**/
            long end = solve(value.substr(pos, i - pos), false, true); // /*/ (all(i - pos) -1) /**/

            res += start * end;
            if (debug) cout << value[i] << " i: " << i << " res: " << res << ", pos: " << pos << ", start: " << start << ", end: " << end << "\n";
        } else {
            res *= 2;
            if (debug) cout << value[i] << ": " << res << "\n";
        }
    }

    if (fixUnique) res--;
    return res;
}

int main() {
    string str;
    cin >> str;
    cout << (solve(str, false) % (long) (pow((long) 10, (long) 9) + 7));

    return 0;
}

Test 1

Group: 1, 2, 3

Verdict: ACCEPTED

Test 2

Group: 1, 2, 3

Verdict: ACCEPTED

Test 3

Group: 1, 2, 3

Verdict: WRONG ANSWER

Test 4

Group: 1, 2, 3

Verdict: ACCEPTED

Test 5

Group: 2, 3

Verdict: TIME LIMIT EXCEEDED

Test 6

Group: 3

Verdict: TIME LIMIT EXCEEDED

Test 7

Group: 3

Verdict: TIME LIMIT EXCEEDED