Package g2401_2500.s2484_count_palindromic_subsequences

Class Solution

java.lang.Object
g2401_2500.s2484_count_palindromic_subsequences.Solution
public class Solution extends Object
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.

  • Constructor Details

    • Solution

      public Solution()

  • Method Details

    • countPalindromes

      public int countPalindromes(String s)