940. Distinct Subsequences II

Problem

Given a string s, return **the number of *distinct non-empty subsequences* of** s. Since the answer may be very large, return it modulo 10^9 + 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.

Solution

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;
    }
}

Explain:

nope.

Complexity:

• Time complexity : O(n).
• Space complexity : O(n).