4Sum
<p>Given an array <code>nums</code> of <code>n</code> integers, return <em>an array of all the <strong>unique</strong> quadruplets</em> <code>[nums[a], nums[b], nums[c], nums[d]]</code> such that:</p>

<ul>
	<li><code>0 &lt;= a, b, c, d&nbsp;&lt; n</code></li>
	<li><code>a</code>, <code>b</code>, <code>c</code>, and <code>d</code> are <strong>distinct</strong>.</li>
	<li><code>nums[a] + nums[b] + nums[c] + nums[d] == target</code></li>
</ul>

<p>You may return the answer in <strong>any order</strong>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> nums = [1,0,-1,0,-2,2], target = 0
<strong>Output:</strong> [[-2,-1,1,2],[-2,0,0,2],[-1,0,0,1]]
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> nums = [2,2,2,2,2], target = 8
<strong>Output:</strong> [[2,2,2,2]]
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= nums.length &lt;= 200</code></li>
	<li><code>-10<sup>9</sup> &lt;= nums[i] &lt;= 10<sup>9</sup></code></li>
	<li><code>-10<sup>9</sup> &lt;= target &lt;= 10<sup>9</sup></code></li>
</ul>
Array; Two Pointers; Sorting
class Solution {
    /**
     * @param int[] $nums
     * @param int $target
     * @return int[][]
     */

    function fourSum($nums, $target) {
        $result = [];
        $n = count($nums);

        sort($nums);

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

            for ($j = $i + 1; $j < $n - 2; $j++) {
                if ($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) {
                        $result[] = [$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--;
                        }

                        $left++;
                        $right--;
                    } elseif ($sum < $target) {
                        $left++;
                    } else {
                        $right--;
                    }
                }
            }
        }
        return $result;
    }
}
