Ositus

#include <bits/stdc++.h>

typedef long long ll;

#define M 1000000007
#define N (1 << 18)

using namespace std;

ll dp[1000001];

int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
    string s;
    cin >> s;
    int n = s.size();

    dp[0] = 1;

    for (int i = 1; i <= n; ++i) {
        set<char> seen;
        for (int j = i; j >= 1; j--) {
            if (seen.find(s[j - 1]) != seen.end()) break;
            seen.insert(s[j - 1]);
            dp[i] = (dp[i] + dp[j - 1]) % M;
        }
    }

    cout << dp[n] << "\n";

    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: 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