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 (1.42.0) (judge verdict: Accepted)

use std::char::from_u32;
use std::collections::HashMap;
fn main() {
    let MOD: u128 = 10u128.pow(9) + 7;
    let n = {
        let mut buf = String::new();
        std::io::stdin().read_line(&mut buf).unwrap();
        let n: u32 = buf
            .trim_end()
            .to_owned()
            .split_whitespace()
            .next()
            .unwrap()
            .parse()
            .unwrap();
        n
    };
    let s = {
        let mut buf = String::new();
        std::io::stdin().read_line(&mut buf).unwrap();
        let s = buf.trim_end().to_owned();
        s
    };

    let mut map: HashMap<char, u128> = HashMap::new();
    for i in 0..26 {
        let c = from_u32('a' as u32 + i).unwrap();
        map.insert(c, 0);
    }

    for c in s.chars() {
        map.insert(c, map.get(&c).unwrap() + 1);
    }
    let mut ans: u128 = 1;
    for v in map.values() {
        ans *= *v + 1;
        ans %= MOD;
    }
    println!("{}", (ans - 1) % MOD)
}