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 <string.h>
#include <math.h>
#include <stdlib.h>
#define MOD 1000000007
#define MAX(a,b) (a>b?a:b)
#define MAX3(a,b,c) MAX(a,MAX(b,c))
#define MIN(a,b) (a<b?a:b)
#define MIN3(a,b,c) MIN(a, MIN(b,c))
typedef long long lli;

int main()
{
    int n, i;
    char s[100000];
    int count[26] = {0};
    scanf("%d",&n);
    scanf("%s",s);
    for(i=0; i<n; i++) {
        char a[] = "a";
        int temp = strncmp(s+i, a, 1);
        count[temp]++;
    }
    lli ans = 1;
    for(i=0; i<26; i++) {
        ans = (lli)ans * (count[i]+1) % MOD;
    }
    ans = (ans==0?MOD-1:ans-1);
    printf("%lld",ans);

    return 0;
}