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>
#include <stdlib.h>
#include <string.h>
#define NUM 1000000007 //10^9+7
#define ll long long int

int main(void){
	ll n,i,ans=1;
	char s[100001];
	scanf("%lld %s",&n,s);
	int alph[26]={0};
	for(i=0;i<n;i++)alph[s[i]-'a']++;
	for(i=0;i<26;i++){
		ans *= (alph[i]+1);
		ans = ans%NUM;
	}
	ans = (ans-1)%NUM;
	printf("%lld",ans);
	return 0;
}