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.*;

public class Main {
    public static void main(String args[]) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        char[] cs = sc.next().toCharArray();
        long[] ls = new long[26];
        long t = 1;
        for(int i = 0 ;i < 26;++i){
            ls[i] = 1;
        }
        for(int i = 0;i < n;++i){
            ++ls[cs[i] - 'a'];
        }
        int c = 0;
        for(int i = 0 ;i < 26;++i){
            t *= ls[i];
            if(ls[i] == 1)++c;
        }
        --t;
        if(c == 25)t = 1;
        t %= 1000000007;
        System.out.println(t);
    }

}