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 Kotlin (1.0.0) (judge verdict: Wrong Answer)

fun main(args: Array<String>){
    val len = readLine()!!.toInt()
    val s = readLine()!!
    val lc = mutableListOf<Char>()
    for(i in 0..(len - 1)){
        lc.add(s[i])
    }
    lc.sort()
    var ret = 1L
    var i = 0
    while(i < len){
        val c = lc[i]
        var cnt = 1L
        while(++i < len - 1){
            if(lc[i] != c) break
            cnt++
        }
        ret = (ret * (cnt + 1L)) % 1000000007L
    }
    println(ret - 1)
}