<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>

class Solution {
public:
    vector<vector<int>> permuteUnique(vector<int>& nums) {
        vector<vector<int>> ret;
        if (nums.size() < 2) {
            ret.push_back(nums);
            return ret;
        }
        unordered_set<int> visited;
        for (int i = 0; i < nums.size(); ++i) {
            if (visited.count(nums[i]) > 0) continue;
            visited.insert(nums[i]);

            vector<int> sub(nums.begin() + 1, nums.end());
            if (i > 0) {
                sub[i-1] = nums[0];
            }
            vector<vector<int>> subPermutations = permuteUnique(sub);
            for (auto sp: subPermutations) {
                sp.insert(sp.begin(), nums[i]);
                ret.push_back(sp);
            }
        }
        return ret;
    }
};
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