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.<p>Hard</p> <p>Given a string of digits <code>s</code>, return <em>the number of <strong>palindromic subsequences</strong> of</em> <code>s</code> <em>having length</em> <code>5</code>. Since the answer may be very large, return it <strong>modulo</strong> <code>10<sup>9</sup> + 7</code>.</p> <p><strong>Note:</strong></p> <ul> <li>A string is <strong>palindromic</strong> if it reads the same forward and backward.</li> <li>A <strong>subsequence</strong> is a string that can be derived from another string by deleting some or no characters without changing the order of the remaining characters.</li> </ul> <p><strong>Example 1:</strong></p> <p><strong>Input:</strong> s = &ldquo;103301&rdquo;</p> <p><strong>Output:</strong> 2</p> <p><strong>Explanation:</strong> There are 6 possible subsequences of length 5: &ldquo;10330&rdquo;,&ldquo;10331&rdquo;,&ldquo;10301&rdquo;,&ldquo;10301&rdquo;,&ldquo;13301&rdquo;,&ldquo;03301&rdquo;. Two of them (both equal to &ldquo;10301&rdquo;) are palindromic.</p> <p><strong>Example 2:</strong></p> <p><strong>Input:</strong> s = &ldquo;0000000&rdquo;</p> <p><strong>Output:</strong> 21</p> <p><strong>Explanation:</strong> All 21 subsequences are &ldquo;00000&rdquo;, which is palindromic.</p> <p><strong>Example 3:</strong></p> <p><strong>Input:</strong> s = &ldquo;9999900000&rdquo;</p> <p><strong>Output:</strong> 2</p> <p><strong>Explanation:</strong> The only two palindromic subsequences are &ldquo;99999&rdquo; and &ldquo;00000&rdquo;.</p> <p><strong>Constraints:</strong></p> <ul> <li><code>1 <= s.length <= 10<sup>4</sup></code></li> <li><code>s</code> consists of digits.</li> </ul>

  • Constructor Details

    • Solution

      public Solution()

  • Method Details

    • countPalindromes

      public int countPalindromes(String s)