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[] nums=new char[n+1];
        nums[n]='0';
        String str=sc.next();
        for(int i=0;i<n;i++){
            nums[i]=str.charAt(i);
        }
        Arrays.sort(nums);
        int tmp=1;
        final int MOD=1000000007;
        long output=1;
        for(int i=1;i<n+1;i++){
            if(nums[i]==nums[i-1]){
                tmp++;
            }else{
                output*=tmp+1;
                output=output%MOD;
                tmp=1;
            }
            
        }
        System.out.println(output==0?MOD-1:output-1);

    }

}