class Solution {
public:
    vector<vector<int>> merge(vector<vector<int>>& intervals) {
        int len = intervals.size();
        if (!intervals.size()) return {};
        sort(intervals.begin(), intervals.end());
        vector<vector<int>> ret{intervals[0]};
        for (int i = 1; i < len; i++) {
            if (ret[ret.size()-1][1] >= intervals[i][0]) {
                ret[ret.size()-1][1] = intervals[i][1] > ret[ret.size()-1][1] ? intervals[i][1] : ret[ret.size()-1][1];
            } else {
                ret.push_back(intervals[i]);
            }
        }
        return ret;
    }
};