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 <iostream>
#include <vector>
#include <algorithm>
#include <unordered_map>
#include <string>
#include <typeinfo>
#include <list>
#include <queue>
#include <stack>
#include <limits.h>
#include <set>
#include <map>
using namespace std;


int main() {
    int N;
    string S;
    cin>>N>>S;
    map<char, long long> m;
    for (char s:S) {
        m[s]++;
    }
    long long ans = 1, mod = 1e9+7;
    for (auto e:m) {
        if (e.second > 1) {
            ans = (e.second + 1) * ans % mod;
        }
        else {
            ans = ans * 2 % mod;
        }
    }
    cout<<(ans-1) % mod<<endl;
    return 0;
}