sort(intervals.begin(), intervals.end()); //need to sort it first
//l and r stores our current interval, starting with the first in the array
int l = intervals[0].first, r = intervals[0].second;
for (int i=1; i<intervals.size();  i++) {
    if (intervals[i].first > r) {
       //this condition means interval 'i' is out of our current (l, r) interval, so our old (l,r) is done
       merged.push_back(make_pair(l, r));
       // We start a new interval with information from intervals[i]:
       l = intervals[i].first; r = intervals[i].second;
    } else {
        //else, it means that the interval intersects (because it's sorted), so we just update the 'r' value with the maximum, merging the interval (l,r) with intervals[i]
       r = max(r, intervals[i].second);
    }
    merged.push_back(make_pair(l,r)); //push the last ones we were checking
}