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;
using System.Collections.Generic;
using System.Linq;

namespace competitive_programming
{
    class Program
    {
        static void Main(string[] args)
        {
            var main = new Program();
            main.solve();
        }

        void solve()
        {
            int N = int.Parse(Console.ReadLine());
            string S = Console.ReadLine();

            Dictionary<char, int> dic = new Dictionary<char, int>();

            for(int i = 0;i < N;i++)
            {
                if (dic.ContainsKey(S[i])) dic[S[i]]++;
                else dic.Add(S[i], 1);
            }

            int score = 1;
            foreach(var key in dic.Keys)
            {
                score = score * (dic[key] + 1);
            }
            Console.WriteLine((score - 1) % (Math.Pow(10, 9) + 7));
        }
    }

    struct Vector2
    {
        public Vector2(int x,int y)
        {
            this.x = x;
            this.y = y;
        }
        public int x;
        public int y;
    }

    struct Vector3
    {
        public int x;
        public int y;
        public int z;
    }
}