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# (.NET Core 3.1.201) (judge verdict: Accepted)

using System;
using System.Text;
using System.Collections.Generic;
using System.Collections;
using static System.Math;
using System.Linq;
using static System.Array;

namespace ConsoleApp2
{
    class Program
    {
        static int N;
        static long mod = 1000000007;
        
        static void Main(string[] args)
        {
            N = int.Parse(Console.ReadLine());
            //R = Console.ReadLine().Split(' ').Select(x => long.Parse(x)).ToArray();
            var S = Console.ReadLine();

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

            long ans = 1;
            foreach (var item in dic)
            {
                var tmp = item.Value;
                ans *= (tmp + 1);
                ans %= mod;
            }

            Console.WriteLine(ans - 1);
        }


    }
}