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 Go (1.6) (judge verdict: Runtime Error)

package main

import (
	"bufio"
	"io"
	"os"
	"strconv"
)

const mod = 1e9 + 7

func main() {
	b := [26]int{}
	rd := bufio.NewReader(os.Stdin)
	var (
		by  byte
		err error
	)
	for err != io.EOF {
		by, err = rd.ReadByte()
		b[by-'a']++
	}

	r := 1
	for i := 0; i < 26; i++ {
		r = r * (b[i] + 1) % mod
	}
	os.Stdout.Write([]byte(strconv.Itoa(r-1) + "\n"))
}