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 Perl (v5.18.2) (judge verdict: Accepted)

#!/usr/bin/perl
my ($n) = map { $_ - 0 } split(/\s+/,<STDIN>);
my $s = <STDIN>;
my $mb = 10**9 + 7;
my %cc = ();
for(my $i=0;$i<$n;$i++){
  my $c = substr($s,$i,1);
  $cc{$c} ++ ;
}
my $ct = 0;
my $p = 0;
foreach my $c (keys %cc){
  $p = ( $p + $cc{$c} + ( $p * $cc{$c} ) ) % $mb;
}
print "$p\n";