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 Java8 (OpenJDK 1.8.0) (judge verdict: Wrong Answer)

import java.util.Scanner;

public class Main {
    static int count = 0;
    private StringBuilder output = new StringBuilder();
    private final String inputstring;
    public Main( final String str ){
        inputstring = str;
    }

    public static void main (String args[])
    {
        Scanner scan = new Scanner(System.in);
        int N = scan.nextInt();
        String S = scan.next();
        Main combobj= new Main(S);
        combobj.combine();
        System.out.println(count-1);
        scan.close();
    }
    
    public void combine() { combine( 0 ); }
    private void combine(int start ){
        count++;
        for( int i = start; i < inputstring.length(); ++i ){
            output.append( inputstring.charAt(i) );
            // System.out.println( output );
            if ( i < inputstring.length() )
            combine( i + 1);
            output.setLength( output.length() - 1 );
        }
    }
}