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

namespace AGC031
{
    class A
    {
        static int _Mod = 1000000007;

        static void Main(string[] args)
        {
            int[] h = Console.ReadLine().Split(' ').Select(z => int.Parse(z)).ToArray();
            int N = h[0];
            string S = Console.ReadLine();

            Dictionary<char, int> dic = new Dictionary<char, int>();
            for(int i = 0; i < S.Length; i++)
            {
                if (!dic.ContainsKey(S[i])) dic.Add(S[i], 0);
                dic[S[i]]++;
            }

            long sum = 1;
            foreach(int val in dic.Values)
            {
                sum *= val+1; sum %= _Mod;
            }

            sum--;
            if (sum < 0) sum += _Mod;

            Console.WriteLine(sum);
        }
    }
}