15. 四数之和

<?php
class Solution {
    public function fourSum($nums, $target) {
        if (!$nums) return [];
        sort($nums);
        $ret = [];
        for($i = 0; $i < count($nums) - 3; $i++) {
            if ($i > 0 && $nums[$i] == $nums[$i - 1]) continue;
            for($j = $i + 1; $j < count($nums) - 2; $j++) {
                if ($j > 0 && $nums[$j] == $nums[$j - 1]) continue;
                $left = $j + 1;
                $right = count($nums) - 1;
                while ($left < $right) {
                    $sum = $nums[$i] + $nums[$j] + $nums[$left] + $nums[$right];
                    if ($sum == $target) {
                        array_push($ret, [$nums[$i] ,$nums[$j] ,$nums[$left] ,$nums[$right]]);
                        while ($left < $right && $nums[$left] == $nums[$left + 1])
                            $left++;
                        while ($left < $right && $nums[$right] == $nums[$right - 1])
                            $right--;
                    } else if ($sum > $target) {
                        $right--;
                    } else {
                        $left++;
                    }
                }
            }
        }
        return $ret;
    }
}
$nums = [1, 0, -1, 0, -2, 2];
$cls = new Solution();
$r = $cls->fourSum($nums, 0);
print_r($r);