Problem:
Score : 200 points

Problem Statement

You are given a string S of length N.
Among its subsequences, count the ones such that all characters are different, modulo 10^9+7. Two subsequences are considered different if their characters come from different positions in the string, even if they are the same as strings.

Here, a subsequence of a string is a concatenation of one or more characters from the string without changing the order.

Constraints

1 \leq N \leq 100000

S consists of lowercase English letters.

|S|=N

Input

Input is given from Standard Input in the following format:

N
S

Output

Print the number of the subsequences such that all characters are different, modulo 10^9+7.

Sample Input 1

4
abcd

Sample Output 1

15

Since all characters in S itself are different, all its subsequences satisfy the condition.

Sample Input 2

3
baa

Sample Output 2

5

The answer is five: b, two occurrences of a, two occurrences of ba. Note that we do not count baa, since it contains two as.

Sample Input 3

5
abcab

Sample Output 3

17

Solution in Rust (1.15.1) (judge verdict: Accepted)

macro_rules! read_line {
    () => {{
        let mut line = String::new();
        std::io::stdin().read_line(&mut line).unwrap();
        line.trim_right().to_owned()
    }};
}

fn main() {
    let m = 1_000_000_007u64;
    read_line!();
    let s = read_line!();
    let mut ab = vec![0u64; 26];
    for c in s.bytes() {
        ab[(c - 'a' as u8) as usize] += 1;
    }
    let mut ans = 1u64;
    for c in 0..26 {
        ans = (ans * (ab[c] + 1)) % m;
    }
    ans = (ans + m - 1) % m;
    println!("{}", ans);
}