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: Runtime Error)

using System;
using System.Collections.Generic;
using System.Collections;
using System.Linq;
using static System.Console;

namespace Code_test
{
    class Program
    {
        static void Main(string[] args) {
            ReadLine();
            string ABC = "abcdefghijklmnopqrstuvwxyz";
            var list = new List<char>(ReadLine().ToCharArray()).ToList();
            Dictionary<char, int> Dict = new Dictionary<char, int>();
            foreach (char e in ABC) Dict.Add(e, 0);
            foreach (char e in list) Dict[e]++;
            Decimal Result = 1;
            foreach (var e in Dict) {
                Result *= e.Value + 1;
                //Result %= (Decimal)Math.Pow(10, 7);
            }
            WriteLine((Result - 1) % (Decimal)Math.Pow(10, 7));

            ReadLine();
        }
    }
}