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 n;
  long mod = 1000000007;
  scanf("%d\n",&n);
  char a[n];
  int b[26];
  for(int i = 0; i < 26; ++i) b[i] = 0;
  for(int i = 0; i < n; ++i) scanf("%c",&a[i]);
  for(int i = 0; i < n; ++i){
    b[a[i] - 'a']++;
  }
  long answer = 1;
  for(int i = 0; i < 26; ++i){
    answer *= (1 + (long)b[i]);
    answer = answer & mod;
  }
  answer--;
  answer = answer % mod;
  printf("%ld\n",answer);
  return 0;
}