<h2><a href="https://leetcode.com/problems/4sum/">18. 4Sum</a></h2><h3>Medium</h3><hr><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>-10⁹ &lt;= nums[i] &lt;= 10⁹</code></li>

<li><code>-10⁹ &lt;= target &lt;= 10⁹</code></li>

</ul>