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 (1.3.71) (judge verdict: Wrong Answer)

import kotlin.String

fun main(args: Array<String>) {
    val n = readInt()
    val s = readString()

    val cnt = MutableList(26) { 0 }
    s.forEach {
        cnt[it - 'a']++
    }
    var ans = 1L
    cnt.forEach {
        if (it > 0) {
            ans *= (it + 1) % 1000000007L
        }
    }
    println(ans - 1)
}

// Tools
fun readString() = readLine()!!
fun readInt() = readLine()!!.toInt()