[LeetCode] 940. Distinct Subsequences II

题：https://leetcode.com/problems/distinct-subsequences-ii/description/

题目

Given a string S, count the number of distinct, non-empty subsequences of S .

Since the result may be large, return the answer modulo 10^9 + 7.

Example 1:

Input: "abc"
Output: 7
Explanation: The 7 distinct subsequences are "a", "b", "c", "ab", "ac", "bc", and "abc".

Example 2:

Input: "aba"
Output: 6
Explanation: The 6 distinct subsequences are "a", "b", "ab", "ba", "aa" and "aba".

Example 3:

Input: "aaa"
Output: 3
Explanation: The 3 distinct subsequences are "a", "aa" and "aaa".

Note:

1. S contains only lowercase letters.
2. 1 <= S.length <= 2000

题目大意

求出字符串中所有不重复的子序列字符串 的数目。

思路

资料：https://www.jianshu.com/p/02501f516437

写的特别好。

class Solution {
    static final int MOD = 1000000007;
    public int distinctSubseqII(String S) {
        int Slen = S.length();
        int[] dp = new int[26];
        for(int i = 1 ;i <=Slen;i++){
            int tailWithChar = 0;
            for(int j = 0;j<26;j++)
                tailWithChar = (tailWithChar+dp[j])%MOD;
            dp[S.charAt(i-1)-'a'] = (tailWithChar+1)%MOD;
        }
        int res = 0;
        for(int j = 0 ;j <26;j++)
            res = (res + dp[j])%MOD;
        return res;
    }
}