这一题不能用差分数组，因为 [1 , 4] [5, 6] 这个就会被合并，正确的做法就是先排序

bool cmp(vector<int> a, vector<int> b) {
    if (a[0] < b[0]) return 1;
    if (a[0] == b[0]) return a[1] < b[1];
    return 0;
}
class Solution {
public:
    vector<vector<int>> merge(vector<vector<int>>& intervals) {
        sort(intervals.begin(), intervals.end());
        int l = intervals[0][0], r = intervals[0][1];
        vector<vector<int>> ans;
        for (int i = 1; i < intervals.size(); i++) {
            int ll = intervals[i][0], rr = intervals[i][1];
            if (ll > r) {
                ans.push_back({ l,r });
                l = ll, r = rr;
            }
            else if (ll <= r) {
                if (rr <= r) {
                    continue;
                }
                else if (rr > r) {
                    r = rr;
                }
            }
        }
        ans.push_back({ l,r });
        return ans;
    }
};