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: Accepted)

#include <bits/stdc++.h>
#include <iomanip>
#define rep(i,n)for(long long i=0;i<(n);i++)
using namespace std;
typedef long long ll;
 
const int MOD=1000000007;
const int INF=0x3f3f3f3f;
const ll INFL=0x3f3f3f3f3f3f3f3f;

ll gcd(ll x, ll y) {
	return y ? gcd(y, x % y) : x;
}
ll lcm(ll x,ll y){
  ll g=gcd(x,y);
  return x/g*y;
}

int main(){
  cin.tie(nullptr);
  ios::sync_with_stdio(false);
  
  int n;
  string s;
  cin >>n>>s;
  int a[26]={};
  ll ans=1;
  rep(i,n){
    a[s[i]-'a']++;
  }
  rep(i,26){
    a[i]++;
    a[i]%=MOD;
    ans*=a[i];
    ans%=MOD;
  }
  ans--;
  ans%=MOD;

  cout<<ans<<endl;
}