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 <queue>
#include <map>
#include <algorithm>
#include <set>
using namespace std;

#define lli long long int
#define REP(i,n)for(int i=0;i<n;i++)
#define LIM 1000000007
 
int main(){

	lli n;
	string s;

	cin>>n;
	cin>>s;
	vector<lli> a(27,0);
	REP(i,n)a[s.at(i)-'a']++;
	
	lli sum=1;

	REP(i,27){
		sum *= (a[i]+1);
		sum %= LIM;
	}
	cout<<sum-1<<endl;

	return 0;
}