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)

#define D 1000000007L
main()
{
    long long int N, ab[26]={0}, ans=0, n=0, j=0;
    char S[100000+1];
    scanf("%lld%s", &N, S);
    for(int i=0; i<N; i++)ab[S[i]-0x61]++;
for(int i=0; i<26; i++)printf("ab[%c]=%d\n", i+0x61, ab[i]);
    ans = (1<<N)-1;
    for(int i=0; i<26; i++)
    {
        if(ab[i]>1)
        {
            j+=ab[i];
            n++;
        }
    }
    ans -= ((1<<j)-2);
printf("n=%lld\n", n);
    printf("%lld", ans%D);
}