<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>

vector<vector<int>> generate(vector<int>& nums, unordered_map<int, int>& m) {
    vector<vector<int>> res;
    for (auto i=m.begin(); i!=m.end(); i++) {
        if (i->second != 0) {
            i->second--;
            for (vector<int> p:generate(nums, m)) {
                p.push_back(i->first);
                res.push_back(p);
            }
            i->second++;
        }
    }

    vector<int> empty_vec;
    vector<vector<int>> empty_vec_vec;
    empty_vec_vec.push_back(empty_vec);
    return res.empty()? empty_vec_vec : res;
}

class Solution {
public:
    vector<vector<int>> permuteUnique(vector<int>& nums) {
        unordered_map<int, int> m;
        for (int n:nums) {
            m[n]++;
        }

        return generate(nums, m);
    }
};
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