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.Linq;

namespace _20190316
{
    class Program
    {
        private static readonly ulong mod = 1000000007;
        static void Main(string[] args)
        {
            var n = int.Parse(Console.ReadLine());
            var str = Console.ReadLine().Trim();
            var distinctedStr = str.Distinct();
            ulong result = 1;
            foreach (char item in distinctedStr)
            {
                result *= (ulong)str.Count(s => s == item) + 1;
            }
            Console.WriteLine((result - 1) % mod);
        }
    }
}