一、题目

Given a string s, return the number of distinct non-empty subsequences of s. Since the answer may be very large, return it modulo 109 + 7.

A subsequence of a string is a new string that is formed from the original string by deleting some (can be none) of the characters without disturbing the relative positions of the remaining characters. (i.e., “ace” is a subsequence of “abcde” while “aec” is not.

Example 1:

Input: s = “abc”
Output: 7
Explanation: The 7 distinct subsequences are “a”, “b”, “c”, “ab”, “ac”, “bc”, and “abc”.
Example 2:

Input: s = “aba”
Output: 6
Explanation: The 6 distinct subsequences are “a”, “b”, “ab”, “aa”, “ba”, and “aba”.
Example 3:

Input: s = “aaa”
Output: 3
Explanation: The 3 distinct subsequences are “a”, “aa” and “aaa”.

Constraints:

1 <= s.length <= 2000
s consists of lowercase English letters.

二、题解

class Solution {
public:int distinctSubseqII(string s) {int mod = 1e9+7;int n = s.size();vector<int> cnt(26,0);int res = 1;for(int i = 0;i < n;i++){int newAdd = (res - cnt[s[i] - 'a'] + mod) % mod;res = (res + newAdd) % mod;cnt[s[i] - 'a'] = (cnt[s[i] - 'a'] + newAdd) % mod;}return (res - 1 + mod) % mod;}
};