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 Java8 (OpenJDK 1.8.0) (judge verdict: Accepted)

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.Scanner;

public class Main {

	public static void main(final String[] args) {
		new Main().run();
	}

	public void run() {
		final Scanner scanner = new Scanner(System.in);
		final int counter = scanner.nextInt();
		scanner.nextLine();
		final char[] charSequence = scanner.nextLine().toCharArray();

		final Map<Integer, MyLong> wordCount = new HashMap<>();
		for (int index = 0; index < counter; index++) {
			wordCount.computeIfAbsent(Integer.valueOf(charSequence[index]) % 26, e -> new MyLong()).increment();
		}

		final List<MyLong> longs = new ArrayList<>(wordCount.values());

		long answer = 1;
		for (MyLong value : longs) {
			answer *= value.getValue() + 1;
			answer %= 1000000007;
		}

		--answer;
		System.out.println(answer);

		scanner.close();

	}

	class MyLong {
		private long value;

		public MyLong() {
			this.value = 0;
		}

		public void increment() {
			this.value++;
		}

		public long getValue() {
			return this.value;
		}
	}

}