FourSum.php

<?php

/**
 * Finds all unique quadruplets in the array which gives the sum of the target.
 * @param Integer[] $nums - An array of integers.
 * @param Interger $target - The target sum.
 * @return Interger[][] - A list of quadruplets that sum up to the target.
 */
class FourSum
{
    public function fourSum(array $nums, int $target): array
    {

        sort($nums);
        $result = [];
        $n = count($nums);

        for($i = 0; $i < $n - 3; $i++) {
            // Avoid duplicates
            if ($i > 0 && $nums[$i] == $nums[$i - 1]) {
                continue;
            }

            for($j = $i + 1; $j < $n - 2; $j++) {
                // Avoid duplicates
                if ($j > $i + 1 && $nums[$j] == $nums[$j - 1]) {
                    continue;
                }

                $left = $j + 1;
                $right = $n - 1;

                while($left < $right) {

                    $sum = $nums[$i] + $nums[$j] + $nums[$left] + $nums[$right];

                    if ($sum == $target) {
                        // Found a valid quadruplet
                        $result[] = [$nums[$i], $nums[$j], $nums[$left], $nums[$right]];

                        // Skip duplicate values for the third number
                        while ($left < $right && $nums[$left] == $nums[$left + 1]) {
                            $left++;
                        }
                        // Skip duplicate values for the fourth number
                        while ($left < $right && $nums[$right] == $nums[$right - 1]) {
                            $right--;
                        }

                        // Move the pointers
                        $left++;
                        $right--;

                    } elseif ($sum < $target) {
                        // If the sum is less than the target, move the left pointer to the right
                        $left++;
                    } else {
                        // If the sum is greater than the target, move the right pointer to the left
                        $right--;
                    }


                } // while

            } // for $j

        } // for $i

        return $result;
    }

}