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问题 A: Colorful Subsequence

题目描述

You are given a string S of length N. Among its subsequences, count the ones such that all characters are different, modulo 109+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≤N≤100000
·S consists of lowercase English letters.
·|S|=N

输入
Input is given from Standard Input in the following format:

N
S

输出
Print the number of the subsequences such that all characters are different, modulo 109+7.