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)

import java.lang.Math.pow

fun main(Args: Array<String>) {
    val N = readLine()
    val S = readLine()

    val strings = S!!.toCharArray()

    val array = Array(26) { 0 }
    strings.forEach { array[it.toInt() - 'a'.toInt()]++ }

    var ans = 1L
    array.forEach { ans *= (it + 1) }
    ans--
    ans %= (pow(10.0, 9.0) + 7).toLong()

    println(ans)
}