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
func fourSum(nums []int, target int) (ans [][]int) {
	n := len(nums)
	if n < 4 {
		return
	}
	sort.Ints(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
			}
			k, l := j+1, n-1
			for k < l {
				x := nums[i] + nums[j] + nums[k] + nums[l]
				if x < target {
					k++
				} else if x > target {
					l--
				} else {
					ans = append(ans, []int{nums[i], nums[j], nums[k], nums[l]})
					k++
					l--
					for k < l && nums[k] == nums[k-1] {
						k++
					}
					for k < l && nums[l] == nums[l+1] {
						l--
					}
				}
			}
		}
	}
	return
}