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.*;
import java.math.*;
 
public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		// 整数の入
		int n = sc.nextInt();
		String s = sc.next();
		long bigValue = 1000000007;

		List<String> array = new ArrayList<String>();
		for(int i=0;i<n;i++){
			array.add(s.substring(i,i+1));
		}
		Collections.sort(array);
        // System.out.println(array);

		long sum =1;
		long prevA = 0;
		String firstS =array.get(0);
		String prevS = array.get(0);
		for(int i=1;i<n;i++){
			String a = array.get(i);
			if(a.equals(prevS)){
				sum += 1 + prevA;
				// System.out.println("i="+i + ", sum="+sum);
			}else{
				prevA = sum;
				prevS = a;
				sum+= sum + 1;
				// System.out.println("i="+i + ", sum="+sum);
			}
		}


        System.out.println(sum%bigValue);
 
	}
}