2484. Count Palindromic Subsequences Hard

1/**
2 * [2484] Count Palindromic Subsequences
3 *
4 * 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 10^9 + 7.
5 * Note:
6 * 
7 * 	A string is palindromic if it reads the same forward and backward.
8 * 	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.
9 * 
10 *  
11 * <strong class="example">Example 1:
12 * 
13 * Input: s = "103301"
14 * Output: 2
15 * Explanation: 
16 * There are 6 possible subsequences of length 5: "10330","10331","10301","10301","13301","03301". 
17 * Two of them (both equal to "10301") are palindromic.
18 * 
19 * <strong class="example">Example 2:
20 * 
21 * Input: s = "0000000"
22 * Output: 21
23 * Explanation: All 21 subsequences are "00000", which is palindromic.
24 * 
25 * <strong class="example">Example 3:
26 * 
27 * Input: s = "9999900000"
28 * Output: 2
29 * Explanation: The only two palindromic subsequences are "99999" and "00000".
30 * 
31 *  
32 * Constraints:
33 * 
34 * 	1 <= s.length <= 10^4
35 * 	s consists of digits.
36 * 
37 */
38pub struct Solution {}
39
40// problem: https://leetcode.com/problems/count-palindromic-subsequences/
41// discuss: https://leetcode.com/problems/count-palindromic-subsequences/discuss/?currentPage=1&orderBy=most_votes&query=
42
43// submission codes start here
44
45impl Solution {
46    pub fn count_palindromes(s: String) -> i32 {
47        0
48    }
49}
50
51// submission codes end
52
53#[cfg(test)]
54mod tests {
55    use super::*;
56
57    #[test]
58    fn test_2484() {
59    }
60}
61