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 C# (Mono 4.6.2.0) (judge verdict: Wrong Answer)

using System;
using System.Collections.Generic;

class Program
{
    static void Main(string[] args)
    {
        int len = int.Parse(Console.ReadLine());
        string input = Console.ReadLine();
        int cnt = 1;
        Dictionary<string, int> dic = new Dictionary<string, int>();
        for (int i = 0; i < len; i++) {
            string str = input.Substring(i, 1);
            if (!dic.ContainsKey(str)) {
                dic.Add(str, Count(input, str) + 1);
            }
        }

        foreach(var data in dic) {
            cnt *= data.Value;
        }
        Console.WriteLine((cnt - 1) % (Math.Pow(10, 9) + 7));
    }

    static int Count(string str, string c)
    {
        return str.Length - str.Replace(c, "").Length;
    }
}