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::io::stdin;
use std::collections::HashMap;

fn main() {
    let mut st = String::new();
    stdin().read_line(&mut st).unwrap();
    let _n = st.trim().parse::<usize>().unwrap();
    let mut st = String::new();
    stdin().read_line(&mut st).unwrap();
    let s:Vec<char> = st.trim().chars().collect();
    let mut map:HashMap<&char, i64> = HashMap::new();
    for i in &s {
        let count = map.entry(i).or_insert(1);
        *count += 1;
    };
    let mo = 1000000007;
    let mut result:i64 = 1;
    for (_i, j) in map.iter() {
        result = (result*j)%mo;
    };
    println!("{}", result-1);
}