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 <math.h>

#define TRUE 1
#define FALSE 0

#define MOD 1000000007

typedef struct _words {
	int size;
	char *s;
} words;

int main(void) {
	long int answer = 1;
	int alphabets[26];
	for (int i = 0; i < 26; i++) {
		alphabets[i] = 1;
	}
	words w;
	scanf("%d", &(w.size));
	w.s = (char *)malloc(sizeof(char) * w.size);
	scanf("%s", w.s);
	for (int i = 0; i < w.size; i++) {
		alphabets[(int)(w.s[i]) - 97]++;
	}
	for (int i = 0; i < 26; i++) {
		answer = (answer * alphabets[i]) % MOD;
	}
	answer -= 1;
	if (answer == -1) {
		answer += MOD;
	}
	printf("%ld\n", answer);
	return 0;
}