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 <cstdio>
#include <cstring>

const int N = 2e5 + 10;
const int MOD = 1e9 + 7;

int lst[N], f[N];

int main() {
	int n;
	memset(lst, 0, sizeof lst);
	scanf("%d", &n);
    f[0] = 0;
	for (int i = 1; i<= n; ++i) {
		int a;
		scanf("%d", &a);
		f[i] = f[i - 1];
		if (lst[a] != 0 && lst[a] != i - 1) f[i] =(f[i] + f[lst[a]] + 1) % MOD;
		lst[a] = i;
	}
	printf("%d\n", (f[n] + 1) % MOD);
	return 0;
}