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g2401_2500.s2484_count_palindromic_subsequences.readme.md Maven / Gradle / Ivy

2484\. Count Palindromic Subsequences

Hard

Given a string of digits `s`, return _the number of **palindromic subsequences** of_ `s` _having length_ `5`. Since the answer may be very large, return it **modulo** 109 + 7.

**Note:**

*   A string is **palindromic** if it reads the same forward and backward.
*   A **subsequence** is a string that can be derived from another string by deleting some or no characters without changing the order of the remaining characters.

**Example 1:**

**Input:** s = "103301"

**Output:** 2

**Explanation:** There are 6 possible subsequences of length 5: "10330","10331","10301","10301","13301","03301". Two of them (both equal to "10301") are palindromic.

**Example 2:**

**Input:** s = "0000000"

**Output:** 21

**Explanation:** All 21 subsequences are "00000", which is palindromic.

**Example 3:**

**Input:** s = "9999900000"

**Output:** 2

**Explanation:** The only two palindromic subsequences are "99999" and "00000".

**Constraints:**

*   1 <= s.length <= 104
*   `s` consists of digits.