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<cstdio>
#include<cstdlib>
#include<string>
#include<sstream>
#include<cmath>
#include<numeric>
#include<map>
#include<stack>
#include<queue>
#include<list>
#include<set>
#include<cstring>
using namespace std;
long long int mod = 1e9 + 7;
int main() {
  int n; cin >> n;
  string s; cin >> s;
  vector<long long int> a(26, 0);
  for(int i=0; i<s.size(); i++){
    a[s[i]-'a']++;
  }

  long long int cnt = 1;
  for(int i=0; i<26; i++) {
    cnt *= (a[i]+1);
    cnt %= mod;
  }
  cnt += mod-1;
  cnt %= mod;

  cout << cnt << endl;


  return 0;
}



//EOF