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 {

	public static void main(String[] args) {
		long mod = 1000000007;
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		sc.nextLine();
		String S = sc.nextLine();
		int[] cnt = new int[27];
		for(int i = 0; i < S.length(); i++) {
			cnt [S.charAt(i)-'a'] ++;
		}
		long ans = 1;
		long tmp;
		for(int i = 0; i < 27; i++) {
			ans += ans*cnt[i];
		}
		--ans;
		System.out.println(ans);
	}

}