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: Accepted)

using System;
using static System.Console;

class Program
{
    public static void Main()
    {
	int n = int.Parse(ReadLine());
	int M = 1000000007;

	int N = 'z' - 'a' + 1;
	long[] a = new long[N];

	string s = ReadLine();
	for (int i = 0; i < n; i++)
	{
	    char c = s[i];
	    a[c - 'a']++;
	}

	long x = 1;
	for (int i = 0; i < N; i++)
	{
	    if (a[i] > 0)
		x = x * (a[i] + 1) % M;
	}

	x = (x + M - 1) % M;
	WriteLine(x);
    }
}