Given a string `s`, find the longest palindromic subsequence's length in `s`.

A subsequence is a sequence that can be derived from another sequence by deleting some or no elements without changing the order of the remaining elements.

Example 1:

    Input: s = "bbbab"
    Output: 4
    Explanation: One possible longest palindromic subsequence is "bbbb".

Example 2:

    Input: s = "cbbd"
    Output: 2
    Explanation: One possible longest palindromic subsequence is "bb".

Constraints:

- `1 <= s.length <= 1000`
- `s` consists only of lowercase English letters.