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;
using System.Threading.Tasks;

namespace abc119d
{
    class Program
    {
        static void Main(string[] args)
        {
            var N = ReadInt();
            var S = Console.ReadLine().ToList();
            var ans = 1;
            for (char c = 'a'; c <= 'z'; c++)
            {
                ans *= S.Count(x => x == c) + 1;
            }
            ans -= 1;
            ans %= (int)Math.Pow(10, 9) + 7;
            Console.WriteLine(ans);
#if DEBUG
            Console.ReadKey();
#endif
        }
        private static int ReadInt() => int.Parse(Console.ReadLine());
    }
}