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 <math.h>

int count_char(const char target, const char* s)
{
    int count = 0;
    char* ch = s;
    
    while(*ch != 0){
        if(target == *ch){
            count++;
        }
        ch++;
    }
    return count;
}

int main()
{
    int n;
    char s[100001] = {0};

    if(scanf("%d", &n) < 1){
        return 0;
    }
    if(scanf("%s",s) < 1) {
        return 0;
    }

    int char_counts[26] = {0};
    for(int i = 0; i < sizeof(char_counts) ; i++){
        char_counts[i] = count_char(0x61+i, s);
    }

    long nums = 1;
    for(int i = 0; i < 26 ; i++){
        nums *= char_counts[i]+1;
    }

    nums--;

    long answer = nums % (long)(pow(10,9) + 7);

    printf("%ld\n", answer);

    return 0;
}