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g0901_1000.s0940_distinct_subsequences_ii.readme.md Maven / Gradle / Ivy

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 <= 2000`
*   `s` consists of lowercase English letters.