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++14 (GCC 5.4.1) (judge verdict: Accepted)

#include <bits/stdc++.h>
using namespace std;

typedef long long ll;

const int mod = 1000000007;
const int Maxl = 26;

int n;
int freq[Maxl];

int main()
{
    scanf("%d", &n);
    string s; cin >> s;
    for (int i = 0; i < n; i++)
        freq[s[i] - 'a']++;
    int res = 1;
    for (int i = 0; i < Maxl; i++)
        res = ll(res) * (freq[i] + 1) % mod;
    res = (res - 1 + mod) % mod;
    printf("%d\n", res);
    return 0;
}