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: Time Limit Exceeded)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApp5
{

    class Program
    {
        static void Main(string[] args)
        {
            

            string S = Console.ReadLine();
            int N = int.Parse(S);
            S = Console.ReadLine();
            var list = new List<string>();

            int i = 0;
            while (i < S.Length)
            {
                list.Add(S.Substring(i,1));
                int L = list.Count;
                for(int j=0; j<L; j++)
                {
                  	//Console.WriteLine(list[j]);
                    if (list[j].IndexOf(S.Substring(i,1)) == -1)
                    {
                        list.Add((list[j] + S.Substring(i,1)));
                    }
                }
              i++;
            }
            Console.WriteLine(list.Count % 1000000007);

        }
    }
}