class Solution {
public:
    vector<vector<int>> result;
    vector<int> path;
    void backtracking(vector<int>& nums , int indnx )
    {
      //小于2的不加入结果，至少两个
        if(path.size()>=2) result.push_back(path);
        if(indnx >= nums.size()) return;
    //用set统计每一层相同值元素的使用
        unordered_set<int> uset;
        for(int i =indnx ; i<nums.size(); i++)
        {
          //如果当前值小于上一个，或者这个值在该层用过了，跳过
            if(path.empty() == 0 && nums[i] < path.back() 
                || uset.find(nums[i]) != uset.end()) 
            continue;
            //将该使用过的存入set，标记使用过
            uset.insert(nums[i]);
            path.push_back(nums[i]);
            backtracking(nums,i+1 );
            path.pop_back();
        }
        return;
    }
    vector<vector<int>> findSubsequences(vector<int>& nums) {
        backtracking(nums,0);
        return result;
    }
};

class Solution {
public:
    vector<vector<int>> result;
    vector<int> path;
    void track_back(vector<int>& nums , int indnx)
    {
        if(indnx > nums.size()) return;
        if(path.size() >= 2 )
        {
             result.push_back(path);
        } 
        set<int> my_set;
        for(int i=indnx ; i<nums.size() ; i++)
        {
            if((path.empty()==0&&nums[i]<path.back()) || 
                my_set.find(nums[i])!=my_set.end()) continue;
            my_set.insert(nums[i]);
            path.push_back(nums[i]);
            track_back(nums,i+1);
            path.pop_back();
        }
    }
    vector<vector<int>> findSubsequences(vector<int>& nums) {
        if(nums.size() == 0) return result;
        track_back(nums,0);
        return result;
    }
};