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;
class Program
    {
    	static void Main(string[] args)
    	{
    		// 整数の入力
    		long a = long.Parse(Console.ReadLine());
    		// 文字列の入力
    		string s = Console.ReadLine();
    		//出力
    		long[] count = new long[26];
            foreach (char c in s)
            {
                count[c - 'a'] ++ ;
            }
            long ans = 1;
            foreach (long i in count) ans = (ans * (i + 1)) % 1000000007;
            Console.WriteLine((ans - 1) % 1000000007);
        }
    }
    //mcs Main.cs
    //mono Main.exe