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.Scanner;
import java.util.BitSet;

public class Main{
	
	static final int N = 100000 + 10, mod = 1000000007;
	static int n;
	static int cnt[] = new int[30];
	static String s;
	
	public static void main(String[] args) {
		Scanner inp = new Scanner(System.in);
		n = inp.nextInt();	
		s = inp.next();
		for(int i = 0; i < s.length(); ++i) {
			cnt[ s.charAt(i) - 'a' ] ++;
			//System.out.println(s.charAt(i) + " " + (s.charAt(i) - 'a'));
		}
		long res = 1;
		for(int i = 0; i < 26; ++i) {
			res = res * (cnt[i] + 1) % mod;
		}
		System.out.print((res + mod - 1) % mod);
	}
}