Project Euler Problem 4 (PHP)
A palindromic number reads the same both ways. The largest palindrome made from the product of two 2-digit numbers is 9009 = 91 × 99.
Find the largest palindrome made from the product of two 3-digit numbers.
Script
Execution Time: 1.3709 seconds
$start = 999;
$r = array();
for ($x = $start; $x >= 100; $x--) {
for ($y=$start; $y >= 100; $y--) {
$p = $x*$y;
if ($p == strrev($p)) {
$r[] = $p;
}
}
}
rsort($r, SORT_NUMERIC);
echo $r[0].PHP_EOL;
Hi thanks for your solution, however it tends to slow down on very large numbers (4 digits above)
Here’s my not-so-clean solution, but I THINK it works.
$start = microtime(TRUE);
$palindrome = $palindrome_i = $palindome_j = 0;
for ($i = 999; $i > 99; $i--) {
for ($j = 999; $j > 99; $j--) {
$product = $i * $j;
if ($product == strrev($product)) {
$palindrome_j = $j;
$palindrome = $product;
break;
}
}
if ($palindrome) {
$palindrome_i = $i;
break;
}
}
$palindrome2 = 0;
for ($i = $palindrome_i; $i > $palindrome_j; $i--) {
for ($j = $palindrome_i; $j > $palindrome_j; $j--) {
$product = $i * $j;
if ($product == strrev($product)) {
$palindrome2 = $product;
break;
}
}
if ($palindrome2) {
break;
}
}
echo ($palindrome2) ? $palindrome2 : $palindrome;
$end = microtime(TRUE);
echo '' . number_format($end - $start, 4) . ' seconds';
Just sharing here, I could be wrong.
I haven’t had a chance to run through the code just yet, but it runs beautifully as well as much more efficiently than mine. Thank you!