public class Solution
extends Object
730 - Count Different Palindromic Subsequences\.
Hard
Given a string s, return _the number of different non-empty palindromic subsequences in_ `s`. Since the answer may be very large, return it **modulo** 109 + 7.
A subsequence of a string is obtained by deleting zero or more characters from the string.
A sequence is palindromic if it is equal to the sequence reversed.
Two sequences a1, a2, ... and b1, b2, ... are different if there is some `i` for which ai != bi.
**Example 1:**
**Input:** s = "bccb"
**Output:** 6
**Explanation:** The 6 different non-empty palindromic subsequences are 'b', 'c', 'bb', 'cc', 'bcb', 'bccb'. Note that 'bcb' is counted only once, even though it occurs twice.
**Example 2:**
**Input:** s = "abcdabcdabcdabcdabcdabcdabcdabcddcbadcbadcbadcbadcbadcbadcbadcba"
**Output:** 104860361
**Explanation:** There are 3104860382 different non-empty palindromic subsequences, which is 104860361 modulo 109 + 7.
**Constraints:**
* `1 <= s.length <= 1000`
* `s[i]` is either `'a'`, `'b'`, `'c'`, or `'d'`.