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 (GCC 5.4.1) (judge verdict: Accepted)

#include<stdio.h>

#define mod 1000000007

int main(){
	int n,i,j;
	char s[100000];
	long long ko[2][27]={0};
	long long ans=0;
	scanf("%d%s",&n,s);
	for(i=n-1;i>=0;i--){
		long long tmp=1;
		for(j=0;j<27;j++)ko[i%2][j]=0;
		ko[i%2][s[i]-'a']++;
		for(j=0;j<27;j++){
			ko[i%2][j]+=ko[(i+1)%2][j];
			if(s[i]-j!='a')tmp*=ko[i%2][j]+1;
			tmp=tmp%mod;
		}
		ans+=tmp;
		ans=ans%mod;
	}
	printf("%lld\n",ans);
	return 0;
}