思路：按左边界排序，若相邻两个区间重叠（表现为intervals[i][0] <= end）,则更新最大右边界end，这样就可以合并更多的区间了，整体最优：合并所有重叠的区间。

class Solution {
public:
    static bool cmp(const vector<int> &a, const vector<int> &b){
        return a[0] < b[0];
    }


    vector<vector<int>> merge(vector<vector<int>>& intervals) {
        if(intervals.size() == 1) return intervals;

        sort(intervals.begin(), intervals.end(), cmp);
        vector<vector<int> > res;
        int end = intervals[0][1], begin = intervals[0][0];

        for(int i = 1; i < intervals.size(); i++){
            if(intervals[i][0] <= end){
                end = max(end, intervals[i][1]);
            }
            else{
                res.push_back({begin, end});
                begin = intervals[i][0];
                end = intervals[i][1];
            }

            if(i == intervals.size() - 1) 
                res.push_back({begin, end});
        }

        return res;
    }
};