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>

#define PI 3.14159265358979323846264338327950288419

using namespace std;

const int mod = 1000000007;
int n;
int freq[128];

int main(){
	scanf("%d", &n);
	for(int i = 1; i <= n; i++){
		char c;
		scanf(" %c", &c);
		freq[c]++;
	}
	
	int ans = 1;
	for(int i = 'a'; i <= 'z'; i++){
		ans = 1LL * ans * (1 + freq[i]) % mod;
	}
	ans = (ans + mod - 1) % mod;
	printf("%d\n", ans);
	return 0;
}