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: Wrong Answer)

#include <stdio.h>
#include <stdlib.h>

int main() {
    int i, n, ans = 1;
    char *s;
    int alphabet[26];

    scanf("%d", &n);
    s = (char*)malloc(n*sizeof(char) + 1);
    scanf("%s", s);

    for (i=0; i < 26; i++) {
        alphabet[i] = 0;
    }

    i = 0;
    while(s[i]) {
        alphabet[s[i] - 'a']++;
        i++;
    }

    for (i=0; i < 26; i++) {
        ans = ans * (alphabet[i] + 1);
    }
    ans--;

    while(ans >= 1000000007) {
        ans -= 1000000007;
    }

    printf("%d\n", ans);
    free(s);
    return 0;
}