Package g2401_2500.s2484_count_palindromic_subsequences

Class 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 Detail

      • Solution

        public Solution()

    • Method Detail

      • countPalindromes

        public int countPalindromes​(String s)