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: Accepted)

import java.util.*;
import java.math.BigInteger;

public class Main {
    public static void main(String[] args) throws Exception {
        // Your code here!
        
        Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
        String S = sc.next();
        char[] strArray = S.toCharArray();
        int[] countChar = new int[26];
        for(int i = 0; i < strArray.length; i++){
            countChar[((int)strArray[i]) % 26] ++;
        }
        
        BigInteger total = new BigInteger("1");
        for(int i = 0; i < 26; i++){
            total = total.multiply(new BigInteger(Integer.toString(countChar[i]+1)));
        }
        total = total.subtract(new BigInteger("1"));
        BigInteger ans = total.remainder(new BigInteger(Integer.toString(1000000007)));
        System.out.println(ans);
    }
}