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 <math.h>

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

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

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

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

    long patterns = 1;
    for(int i = 0; i < sizeof(char_counts)/sizeof(char_counts[0]) ; i++){
        patterns *= char_counts[i] + 1;
        patterns %= 1000000007L;
    }

    patterns--;

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

    return 0;
}