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  1. <?php
  2.  
  3. /**
  4.  * Project Euler Problem 4: Largest palindrome product
  5.  * 
  6.  * A palindromic number reads the same both ways. The largest palindrome made 
  7.  * from the product of two 2-digit numbers is 9009 = 91 × 99.
  8.  * 
  9.  * Find the largest palindrome made from the product of two 3-digit numbers.
  10.  * 
  11.  * Problem: https://p...content-available-to-author-only...r.net/problem=4
  12.  * Solution: https://g...content-available-to-author-only...b.com/potherca-blog/ProjectEuler/blob/master/src/PHP/Solutions/Problem004.php
  13.  * Live code: https://i...content-available-to-author-only...e.com/MwUCwn
  14.  */
  15. namespace Potherca\ProjectEuler\Solutions\Problem004
  16. {
  17.     use Potherca\ProjectEuler\Calculators\PalindromeProductCalculator as Calculator;
  18.  
  19. 	$digits = 3;
  20.  
  21.     $solution = (new Calculator())->getHighestPalindromeForProduct($digits);
  22.  
  23.     echo $solution;
  24. }
  25.  
  26. namespace Potherca\ProjectEuler\Calculators
  27. {
  28.     class PalindromeProductCalculator
  29.     {
  30.         final public function getHighestPalindromeForProduct(int $digits): int
  31.         {
  32.         	$palindromes = $this->getPalindromesForProduct($digits);
  33.  
  34.         	return end($palindromes);
  35.         }
  36.  
  37.         final public function getPalindromesForProduct(int $digits): array
  38.         {
  39.         	$low = 10 ** ($digits - 1);
  40.         	$high = (10 ** $digits) - 1;
  41.  
  42.         	return $this->getPalindromesForRange($low, $high);
  43.         }
  44.  
  45.         final public function getPalindromesForRange(int $low, int $high): array
  46.         {
  47. 			$palindromes = [];
  48.  
  49. 			$numbers = range($low, $high);
  50.  
  51. 			foreach ($numbers as $left) {
  52. 				foreach ($numbers as $right) {
  53. 					$number = $left * $right;
  54.  
  55. 					$reverse = (int) strrev($number);
  56.  
  57. 					if (
  58. 						$reverse === $number
  59. 						&& ! in_array($number, $palindromes)
  60. 					) {
  61. 						$palindromes[] = $number;
  62. 					}
  63. 				}
  64. 			}
  65.  
  66. 			sort($palindromes);
  67.  
  68. 			return $palindromes;
  69.         }
  70.     }
  71. }
  72.  
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https://ideone.com/MwUCwn
Solution in PHP for Project Euler Problem 004
language:
PHP (php 7.3.5)
created:
5 years ago
visibility:
public

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