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 (
        "fmt"
        "math"
)

func main() {
        var N int
        fmt.Scan(&N)
        var S string
        fmt.Scan(&S)

        m := map[byte]uint64{}
        for i := 0; i < N; i++ {
                index := S[i]
                _, e := m[index]
                if e {
                        m[index]++
                } else {
                        m[index] = uint64(1)
                }
        }

        var res uint64 = 1
        M := uint64(math.Pow10(9) + 7)

        for _, v := range m {
                res *= (uint64(1) + v)
                res = res % M
        }
        fmt.Print(res - uint64(1))

}