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: Accepted)

package main

import (
	"bufio"
	"fmt"
	"os"
	"strconv"
)

var sc = bufio.NewScanner(os.Stdin)

func nextString() string {
	sc.Scan()
	return sc.Text()
}

func nextInt() int {
	sc.Scan()
	val, err := strconv.Atoi(sc.Text())
	if err != nil {
		panic(err)
	}
	return val
}


// 長い文字列を読み込む時に使う //
var rdr = bufio.NewReaderSize(os.Stdin, 1000000)

func readLine() string {
    buf := make([]byte, 0, 1000000)
    for {
        l, p, e := rdr.ReadLine()
        if e != nil {
            panic(e)
        }
        buf = append(buf, l...)
        if !p {
            break
        }
    }
    return string(buf)
}
/////////////////////////////


func main() {
	sc.Split(bufio.ScanWords)

	readLine()
	xs := readLine()

	dic := make(map[rune]int)

	for _, char := range xs {
		val, ok := dic[char]
		if !ok {
			val = 0
		}
		dic[char] = val + 1
	}
	prod := 1
	div := 1000000007
	for _, val := range dic {
		prod = (prod * (val + 1)) % div
	}
	fmt.Println(prod - 1)
}