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++14 (GCC 5.4.1) (judge verdict: Wrong Answer)

#include <iostream>
#include <string>
#include <map>

using namespace std;

typedef long long ll;

int main(){
  int N;
  string S;
  ll ans = 1;
  ll mod = 1e9 + 7;
  map<char, int> num;
  string al = "abcdefghijklmnopqrstuvwxyz";
  for (int i=0; i<al.size(); i++){
    num[al[i]] = 0;
  }
  cin >> N;
  cin >> S;
  for (int i=0; i<N; i++) num[S[i]]++;
  for (int i=0; i<N; i++){
    ans *= (num[al[i]] + 1);
    ans = ans % mod;
  }
  
  cout << (ans != 0 ? ans - 1 : mod - 1) << endl;
  return 0;
}