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 proconio::{input, marker::Chars};
use std::collections::HashMap;

const MOD: usize = 1_000_000_007;

fn main() {
    input! {
        _n: usize,
        s: Chars,
    }

    let mut ans = 1;

    let mut map = HashMap::new();
    for c in s {
        *map.entry(c).or_insert(0) += 1;
    }

    for (_, val) in map {
        
        ans *= val+1;
        ans %= MOD;
    }

    ans += MOD;
    ans -= 1;
    ans %= MOD;

    println!("{}", ans);
}