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>

char S[100000];

void Do(int N){
  int i;
  int s[32];
  long long c;
  scanf("%s", S);
  c = 1;
  for(i = 0;i <= 'z' - 'a';i++) s[i] = 0;
  for(i = 0;i < N;i++) s[S[i]-'a']++;
  for(i = 0;i <= 'z' - 'a';i++) c = c * (s[i] + 1) % ((long long) (1e9 + 7));
  printf("%lld\n", (c - 1) % ((long long) (1e9 + 7)));  
}

int main(void){
  int N;
  scanf("%d", &N);
  Do(N);
  return 0;
}