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>
#define MOD 1000000007

typedef long long int lli;


#define kake(a,b) ( (a) *(b)%MOD)

int main(void){
  int n,i;
  scanf("%d",&n);
  char s[n];
  scanf("%s",s);
  int a[26]={};

  for(i=0;i<n;i++)
    a[s[i]-'a']++;
  lli num=1;
  for(i=0;i<26;i++){
    num=kake(num,a[i]+1);
  }

  printf("%ld",num-1);
 
  return 0;
}