vector<vector<int>> merge(vector<vector<int>>& intervals) {
    sort(intervals.begin(), intervals.end());
    vector<vector<int>> result;
    for (const auto& vec: intervals) {
        if (result.empty() || result.back()[1] < vec[0]) {
            result.push_back(vec);
        }
        else {
            result.back()[1] = max(result.back()[1], vec[1]);
        }
    }
    return result;
}