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 JavaScript (node.js v5.12) (judge verdict: Wrong Answer)

function roop(depth, A, end){
    var ans=0, i=0
    if (depth < end){
        for (i=depth;i<end;i++){
            ans+=roop(i+1, A, end)
        }
        return A[depth] * (1+ans)
    }else{
        return A[depth]
    }
}

function main(input) {
    var i, j, A, B, ans=0, count=0
    var inputs = input.split("\n")
    var arg = Number(inputs[0])
    var str = inputs[1].split("")
    //A=new Array(arg[0])
    A={}
    for (i=0;i<arg;++i){
        if (A[str[i]]){
            A[str[i]]++
        }else{
            A[str[i]]=1
            count++
        }
    }
    B=[]
    for(key in A){
        B.push(A[key])
    }
    for(i=0;i<count;++i){
        ans+=roop(i, B, count-1)
    }
    console.log(ans%(1000000007))
}
main(require('fs').readFileSync('/dev/stdin', 'utf8'));