LeetCode-in-Java
940. Distinct Subsequences II
Hard
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 <= 2000sconsists of lowercase English letters.
Solution
public class Solution {
public int distinctSubseqII(String str) {
int m = (int) (1e9) + 7;
int prev = 1;
int[] subseqAtPrevIndexOfMyLastOcc = new int[26];
for (int i = 1; i <= str.length(); i++) {
int crntStrIdx = i - 1;
int subtract = subseqAtPrevIndexOfMyLastOcc[str.charAt(crntStrIdx) - 'a'];
int curr = (2 * prev) % m;
curr = (curr + m - subtract) % m;
subseqAtPrevIndexOfMyLastOcc[str.charAt(crntStrIdx) - 'a'] = prev;
prev = curr;
}
return prev - 1;
}
}