class Solution {
private:
    static int compare(const Interval& a, const Interval& b) {
        return a.start < b.start;
    }
public:
    vector<Interval> merge(vector<Interval> &intervals) {
        vector<Interval> res;
        int len = intervals.size();
        if (len < 1) return res;

        sort(intervals.begin(), intervals.end(), Solution::compare);
    
        Interval merged = intervals[0];
        for (int i=1; i<len; i++) {
            Interval& cur = intervals[i];
            if (merged.end >= cur.start) { // merge two intervals
                if (merged.end < cur.end) merged.end = cur.end;
            } else {
                res.push_back(merged);
                merged = cur;
            }
        }
        res.push_back(merged);

        return res;
    }
};