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.Generic;
using System.Linq;
using System.Text;

namespace atcorder_GrandContest_031
{
    class Program
    {

        public const int MOD = 1000000000 + 7;

        static void Main(string[] args)
        {

            int cnt = int.Parse(Console.ReadLine());
            var data = Console.ReadLine().ToArray();

            List<int> charList = new List<int>();
            int[] cntArr = new int[128];

            int charCnt = 0;

            for (int i = 0; i < cnt; i++)
            {
                int aVal = (int)data[i];
                if (0 == cntArr[aVal])
                {
                    charList.Add(aVal);
                    charCnt++;
                }
                cntArr[aVal]++;
            }

            long ret = 1;
            for (int i = 0; i < charCnt; i++)
            {
                ret *= cntArr[charList[i]] + 1;
            }

            ret--;
            ret %= MOD;

            Console.WriteLine(ret);

        }

    }
}