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) {
        int mod = 1000000000 + 7;
        Scanner std = new Scanner(System.in);
        int n = std.nextInt();
        String s = std.next();
        int[] appearCounts = new int[26];
        int index = 0;
        for (char c = 'a'; c <= 'z'; c++) {
            for (int i = 0; i < n; i++) {
                char t = s.charAt(i);
                if (c == t) {
                    appearCounts[index]++;
                }
            }
            index++;
        }

        int ans = 1;
        for (int appearCount : appearCounts) {
            ans *= (appearCount + 1) % mod;
        }

        ans--;

        System.out.println(ans);
    }
}