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.HashMap;
import java.util.Map;
import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		String S = sc.next();
		sc.close();

		char[] ch = S.toCharArray();
		Map<Character, Integer> chMap = new HashMap<>();
		for(int i = 0;i < ch.length;i++) {
			if(chMap.containsKey(ch[i])) {
				chMap.put(ch[i], chMap.get(ch[i])+1);
			}else {
				chMap.put(ch[i], 1);
			}
		}
		int ans = 1;
		for(Map.Entry<Character, Integer> entry : chMap.entrySet()) {
			ans = (ans * (entry.getValue() + 1)) % 1_000_000_007;
		}
		ans--;
		System.out.println(ans);
	}
}