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>
int main (){
  int a,i,j,sum=0,l,k;
  char s[100001];
   scanf ("%d %s",&a,s);
  for (i=0;i<a;i++){
  for (j=i;j<a;j++){
  if(i==j){
    sum=(sum+1)%1000000007;
  }
  else if(s[i] == s[j]){
  break;
  }
  else {
  sum=(sum+1)%1000000007;
  }
  }
  }
  printf ("%d",sum);
return 0;
}