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<vector>

using namespace std;

int alphabet[26];

typedef long long ll;

const int mod = 1e9 + 7;

int main(){
int N; cin >> N;
  for(int i = 0; i < 26; i++){ alphabet[i] = 1;}
  string S; cin >> S;
  
  for(int i = 0; i < N; i++){
    int c = S[i] - 'a'; alphabet[c]++;}
  
  ll sum = 0;
  for(int i = 0; i < 26; i++){
    sum *= alphabet[i] ; sum = sum % mod;}
  cout << sum - 1 << endl; return 0;}