code: public static List<Interval> combine(List<Interval> intervals) {
          List<Interval> combined = new ArrayList<Interval>();
          Collections.sort(intervals);
          int len = intervals.size();
          for (int i = 0; i < len; i++) {
              combined.add(intervals.get(i));
  
              int j;
              for (j = i + 1; j < len; j++) {
                  if (intervals.get(i).stop >= intervals.get(j).start) {
                      intervals.get(i).stop = intervals.get(j).stop;
                  } else {
                      break;
                  }
              }
              i = j - 1;
          }
  
          return combined;
      }
docs: 把{start,stop}连续的合并掉: [{start:1, stop:4},{start:4, stop:5}] => [{start:1,
  stop:5}]
  
  @param intervals
  @return