https://blog.csdn.net/weixin_41404773/article/details/80733324
目标求 0+1+2+3+4+5+....+1000
 初始 start=0 ,end=1000
  middle=(start+end)/2=500

  RaskDemo left = new RaskDemo(start, middle);

RaskDemo right = new RaskDemo(middle, end); right应该变成 RaskDemo right = new RaskDemo(middle+1, end);