Ositus

const M: u64 = 1_000_000_007;

fn main() {
    let mut input = String::new();
    std::io::stdin().read_line(&mut input).unwrap();
    input.pop();

    let arr: Vec<usize> = input.bytes().map(|b| (b - b'a') as usize).collect();
    let mut counts = vec![1_u64];
    
    for i in 0..arr.len() {
        counts.push(0);
        let mut is_in_slice = [false; 26];
        is_in_slice[arr[i]] = true;
        for j in (0..=i).rev() {
            *counts.last_mut().unwrap() += counts[j];
            *counts.last_mut().unwrap() %= M;
            
            if j == 0 || is_in_slice[arr[j - 1]] {
                break;
            }

            is_in_slice[arr[j - 1]] = true;
        }
    }

    println!("{}", counts.last().unwrap());
}

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