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 <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define INF 1e18
#define MOD 1000000007
int main(){
	ll n,ans=0;
	string s;
	cin>>n>>s;
	map<char,ll>sum;
	for(int i=0;i<n;i++){
		sum[s[i]]++;
		ll hoge=1;
		for(auto itr:sum){
			if(itr.first!=s[i]){
				hoge*=itr.second+1;
			}
		}
		ans+=hoge;
		ans%=MOD;
	}
	cout<<ans<<endl;
}