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: Accepted)

using System;
using System.Linq;//リストの使用
using System.Collections.Generic;
using System.Text;//テキストの高速出力に必要
class Program
{
	static void Main()
	{
		int n = int.Parse(Console.ReadLine());
		string s = Console.ReadLine();
    int[] alphabetUses = new int[26];
		long answer = 1;
    long mod = 1000000007;

    for(int i = 0; i < n; i++)
    {
      alphabetUses[s[i]-'a']++;
    }

    for(int i = 0; i < 26; i++)
    {
      if(alphabetUses[i] >= 1)
      {
        answer *= alphabetUses[i]+1;
        answer %= mod;
      }
    }

    answer += mod;
    answer--;
    answer %= mod;

		Console.WriteLine(answer);
	}
}