940. Distinct Subsequences II
Given a string
S
, count the number of distinct, non-empty subsequences ofS
.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:
S
contains only lowercase letters.1 <= S.length <= 2000
Approach #1: Math. [Java]
class Solution { public int distinctSubseqII(String S) { long end[] = new long[26], mod = (long)1e9 + 7; for (char c : S.toCharArray()) end[c - 'a'] = Arrays.stream(end).sum()%mod + 1; return (int)(Arrays.stream(end).sum()%mod); } }
Analysis:
Reference:
永远渴望,大智若愚(stay hungry, stay foolish)