<p>Given an integer array <code>nums</code> of <strong>unique</strong> elements, return <em>all possible</em> <span data-keyword="subset"><em>subsets</em></span> <em>(the power set)</em>.</p>

<p>The solution set <strong>must not</strong> contain duplicate subsets. Return the solution in <strong>any order</strong>.</p>

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

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

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

<pre>
<strong>Input:</strong> nums = [0]
<strong>Output:</strong> [[],[0]]
</pre>

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

<ul>
	<li><code>1 &lt;= nums.length &lt;= 10</code></li>
	<li><code>-10 &lt;= nums[i] &lt;= 10</code></li>
	<li>All the numbers of&nbsp;<code>nums</code> are <strong>unique</strong>.</li>
</ul>

template<typename T>
void print_set(string_view message, const T& collection, string_view end="");

template<typename T>
void print_set(string_view message, const std::set<T>& item_collection, string_view end) {
    std::cout << message <<"[";
    auto it = item_collection.begin();
    if (it != item_collection.end()) {
        std::cout<< *it++;
    }
    for (; it != item_collection.end(); it++) {
        std::cout << "," << *it;
    }
    std::cout << "]" << end;
}

template<>
void print_set(string_view message, const std::set<std::set<int>>& item_collection, string_view end) {
    std::cout << message <<"[";
    auto it = item_collection.begin();
    if (it != item_collection.end()){
        print_set("", *it, "");
        it++;
    }
    for (;it != item_collection.end(); it++) {
        std::cout<<",";
        print_set("", *it, "");
    }
    std::cout << "]" << end;
    
}

class Solution {
     
private:
    void backtrack(int n, const vector<int>& nums, set<int>& cur, set<set<int>>& ans) {
        if (n == cur.size()) {
            // print_set("leaf cur:", cur, "\n");
            return;
        }

        for (const auto i : nums) {
            if (cur.find(i) == cur.end()) {
               cur.insert(i);
               // print_set("push cur:", cur, "\n");
               if (ans.find(cur) == ans.end()) {
                 ans.insert(cur);
                 // print_set("ans:", ans, "\n");
                 backtrack(n, nums, cur, ans);
               }
               // print_set("push cur:", cur, "\n");
               cur.erase(i);
            }
        }
    }
public:
    vector<vector<int>> subsets(vector<int>& nums) {
        set<int> cur;
        set<set<int>> ans;
        int n = nums.size();
        ans.insert(std::set<int>());
        backtrack(n, nums, cur, ans);
        vector<vector<int>> res;
        for (const auto& s : ans) {
            std::vector<int> v(s.begin(), s.end());
            res.push_back(std::move(v));
        }
        return res;
    }
};
10600