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;
use std::io::BufRead;

fn main() {
    let mut sc = Scanner::new(std::io::stdin());
    let n: usize = sc.next().unwrap();
    let s = sc.next_string().unwrap();
    let mut cs = vec![0; 26];
    for c in s.into_bytes().into_iter() {
        cs[(c - 97) as usize] += 1;
    }
    let m = (1e9 as i64) + 7;
    let mut res = 1i64;
    for i in 0..26 {
        res = res * (cs[i] + 1) % m;
    }
    res -= 1;
    if res < 0 {
        res += m;
    }
    println!("{}", res);
}

/// Scanner
///
/// # Example
/// ```
/// use contest::io::scanner;
/// let mut sc = scanner::Scanner::new("1 2 \n\n \r\t \n 3.5".as_bytes());
/// assert_eq!("1".to_string(), sc.next::<String>().unwrap());
/// assert_eq!(2, sc.next());
/// assert_eq!(3.5, sc.next());
/// assert_eq!(None, sc.next::<i32>());
/// ```

pub struct Scanner<R: io::Read> {
    br: io::BufReader<R>,
    // Read tokens are stored in reversed order per line.
    buf: Vec<String>,
}

impl<R: io::Read> Scanner<R> {
    pub fn new(r: R) -> Scanner<R> {
        Scanner {
            br: io::BufReader::new(r),
            buf: vec![],
        }
    }
    #[inline]
    pub fn next<T>(&mut self) -> Option<T>
        where T: std::str::FromStr,
              T::Err: std::fmt::Debug
    {
        self.next_string().map(|s| s.parse::<T>().expect("Parse failed: "))
    }
    fn next_string(&mut self) -> Option<String> {
        self.buf.pop().or_else(|| match self.update() {
            true => self.next_string(),
            false => None,
        })
    }
    #[inline]
    fn update(&mut self) -> bool {
        let mut s = String::new();
        let res = self.br.read_line(&mut s);
        match res.expect("I/O error.") {
            0 => false,
            _ => {
                self.buf = s.split_whitespace().map(|x| x.to_string()).rev().collect();
                true
            }
        }
    }
}