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Java-based LeetCode algorithm problem solutions, regularly updated
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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.
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