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 (Clang 3.8.0) (judge verdict: Accepted)

//https://atcoder.jp/contests/agc031/tasks/agc031_a
#include <iostream>
#include <algorithm>
#include <string>
using namespace std;
const int mod = 1e9 + 7;
int alphabet[30];
long long ans = 1;

int main(int argc, char const *argv[]) {
  int N;
  string S;
  cin >> N >> S;
  for (int i=0; i<S.size(); i++){
    alphabet[S[i] - 'a' + 1]++;
  }
  for (int i=0; i<30; i++){
    ans = ans * (alphabet[i]+1);
    ans = ans%mod;
  }
  cout<<ans-1<<endl;
  return 0;
}