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 PHP7 (7.0.15) (judge verdict: Accepted)

<?php
$dummy = fgets(STDIN);
$a = str_split(trim(fgets(STDIN)));
$s = [];
foreach ($a as $v) {
	if (!isset($s[$v])) {
		$s[$v] = 1;
	} else {
		$s[$v]++;
	}
}

$ans = 1;
foreach ($s as $v) {
	$ans *= ($v + 1);
	$ans %= pow(10, 9) + 7;
}

echo $ans - 1;