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: Wrong Answer)

<?php
$n = trim(fgets(STDIN));
$s = trim(fgets(STDIN));
$max = pow(2, $n) - 1;
for ($i = 0; $i < $n; $i++) {
    if ($i == 0) {
        $count = 1;
        continue;
    }
    $a = '/' . substr($s, $i, 1) . '/';
    $before = substr($s, 0, $i);
    if (preg_match($a, $before) === 1) {
        $count += ($count-1)/2 + 1;
    } else {
        $count = $count * 2 + 1;
    }
}
echo $count % (pow(10, 9) + 7);