how to find the subsequences of an array

vector<vector<int>> findSubsequences(vector<int>& nums) {
        vector<vector<int>> res;
        vector<int> sub;
        subsequence(nums, res, sub, 0);
        return res;
    }
    
    void subsequence(vector<int>& nums, vector<vector<int>>& res, vector<int>& sub, int pos) {
        if(sub.size()>=2){
            res.push_back(sub);
        }
        
        unordered_set<int> hash;
        for(int i=pos; i<nums.size(); i++) {
            if((sub.size() == 0 || nums[i] >= sub.back()) && hash.find(nums[i]) == hash.end()) {
                sub.push_back(nums[i]);
                subsequence(nums, res, sub, i+1);
                sub.pop_back();
                hash.insert(nums[i]);
            }
        }
    }