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>
#define pb push_back
#define mp make_pair
#define int long long
#define ip pair<int, int>

#define N 3000007
#define inf 10000000
#define MOD 1000000007
#define fast ios::sync_with_stdio(0); cin.tie(NULL); cout.tie(NULL)
#define endl "\n"

using namespace std;

int f[100007][26];
signed main()
{
	//fast;
	int n;
	cin>>n;
	string s;
	cin>>s;
	int ans = 0;
	
	memset(f, 0, sizeof(f));
	for(int i=1;i<=n;i++)
	{
		for(int j=0;j<26;j++)
			f[i][j] = f[i-1][j];
		f[i][s[i-1]-'a']++;
	}

	for(int i=1;i<=n;i++)
	{
		int k = 1;
		for(int j=0;j<2;j++)
		{
			if(s[i-1]-'a' != j)
			{
			    //cout<<j<<" "<<f[n-1][j]<<" "<<f[i][j]<<endl;
			    
				k *= (f[n][j] - f[i][j]+1);
				k %= MOD;
			}
		}
		ans += k;
		ans %= MOD;
	}
  	ans += MOD;
  	ans %= MOD;
	cout<<ans;
}