Permutations II
<p>Given a collection of numbers, <code>nums</code>,&nbsp;that might contain duplicates, return <em>all possible unique permutations <strong>in any order</strong>.</em></p>

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

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

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

<pre>
<strong>Input:</strong> nums = [1,2,3]
<strong>Output:</strong> [[1,2,3],[1,3,2],[2,1,3],[2,3,1],[3,1,2],[3,2,1]]
</pre>

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

<ul>
	<li><code>1 &lt;= nums.length &lt;= 8</code></li>
	<li><code>-10 &lt;= nums[i] &lt;= 10</code></li>
</ul>
Array; Backtracking; Sorting
func permuteUnique(nums []int) (ans [][]int) {
	slices.Sort(nums)
	n := len(nums)
	t := make([]int, n)
	vis := make([]bool, n)
	var dfs func(int)
	dfs = func(i int) {
		if i == n {
			ans = append(ans, slices.Clone(t))
			return
		}
		for j := 0; j < n; j++ {
			if vis[j] || (j > 0 && nums[j] == nums[j-1] && !vis[j-1]) {
				continue
			}
			vis[j] = true
			t[i] = nums[j]
			dfs(i + 1)
			vis[j] = false
		}
	}
	dfs(0)
	return
}