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)

use std::cmp::*;
use std::io::*;
const MOD: usize = 1_000_000_007;

fn main() {
    let mut s: String = String::new();
    stdin().read_to_string(&mut s).ok();
    let mut itr = s.trim().split_whitespace();
    let _: usize = itr.next().unwrap().parse().unwrap();
    let s: Vec<char> = itr.next().unwrap().chars().collect();

    let mut a = vec![0; 30];
    for &c in s.iter() {
        let i = (c as u8 - 'a' as u8) as usize;
        a[i] += 1;
    }

    let mut ans = 1;
    for i in 0..26 {
        ans = ans * (a[i] + 1) % MOD;
    }
    ans = (ans + MOD - 1) % MOD;
    println!("{}", ans);
}