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;

long long MOD=1000000007;
vector<long long> c(26,0);

int main(){
	int N;
	string S;
	cin >> N >> S;
	for(int i=0;i<N;i++){
		c[S[i]-'a']++;
	}
	
	long long ans=1;
	for(int i=0;i<26;i++){
		ans*=c[i]+1;
		if(ans>=MOD)ans%=MOD;
	}
	ans+=MOD-1;
	ans%=MOD;
	cout << ans << endl;
	return 0;
}