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;
namespace agc
{
    class Program
    {
        static void Main(string[] args)
        {
            string input = Console.ReadLine();
            int len = input.Length;
            List<string> result = new List<string>();
            Dictionary<int, List<string>> dic = new Dictionary<int, List<string>>();

            for (int i = len - 1; i >= 0; i--) {
                List<string> list = new List<string>();
                string c = input.Substring(i, 1);
                list.Add(c);
                result.Add(c);
                for (int j = i + 1; j <= len - 1; j++) {
                    if (dic.ContainsKey(j)) {
                        var temp = dic[j];
                        foreach (var data in temp) {
                            if (data.IndexOf(c) >= 0) {
                                continue;
                            }
                            var word = c + data;
                            list.Add(word);
                            result.Add(word);
                        }
                    }
                }
                dic.Add(i, list);
            }

            Console.Write(result.Count % (Math.Pow(10, 9) + 7));
            return;

        }
    }
}