// E-RANK → S-RANK RECURSION TEMPLATE
// Pattern: Subsets via Backtracking

void solve(int idx, vector<int>& arr,
           vector<int>& curr,
           vector<vector<int>>& res) {
    res.push_back(curr);

    for (int i = idx; i < arr.size(); i++) {
        curr.push_back(arr[i]);   // choose
        solve(i+1, arr, curr, res); // explore
        curr.pop_back();          // unchoose
    }
}
// Time: O(2^n) | Space: O(n)
// Used by: Tourist, Benq, Um_nik