将所有区间按左端点从小到大排序

1. 无交集 ： 则保存当前区间

2. 有交集 ： 则更新右端点

class Solution {
public:
    vector<vector<int>> merge(vector<vector<int>>& intervals) {
        vector<vector<int>> res;
        sort(intervals.begin(), intervals.end());
        int l = -2e9, r = -2e9;
        for (auto s : intervals)
            if (r < s[0]) {
                if (l != -2e9) res.push_back({l, r});
                l = s[0], r = s[1];
            }
            else r = max(r, s[1]);

        if (l != -2e9) res.push_back({l, r});
        return res;
    }
};

class Solution {
public:
    vector<vector<int>> merge(vector<vector<int>>& a) {
        vector<vector<int>> res;
        if (a.empty()) return res;
        sort(a.begin(), a.end());
        int l = a[0][0], r = a[0][1];
        for (int i = 1; i < a.size(); i ++) {
            if (r < a[i][0]) {
                res.push_back({l, r});
                l = a[i][0], r = a[i][1];
            }
            else r = max(r, a[i][1]);
        }
        res.push_back({l, r});
        return res;
    }
};