<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:
    void solve(vector<int>& nums, int index, vector<vector<int>>& ans) {
        if (index >= nums.size()) {
            ans.push_back(nums); 
            return;
        }

        unordered_set<int> swapped;
        for (int j = index; j < nums.size(); j++) {
            if (swapped.find(nums[j]) != swapped.end()) {
                continue; 
            }

            swapped.insert(nums[j]); 
            swap(nums[index], nums[j]);
            solve(nums, index + 1, ans);
            swap(nums[index], nums[j]);
        }
    }

    vector<vector<int>> permuteUnique(vector<int>& nums) {
        vector<vector<int>> ans;
        sort(nums.begin(), nums.end());
        solve(nums, 0, ans);
        return ans;
    }
};

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