sum明明在前面赋值了  为什么运行结果是sum=2sum=-1sum=3sum=-2sum=4sum=-3sum=5sum=-4sum=6sum=-5sum=7sum=-6sum=8sum=-7sum=9sum=-8sum=10sum=-9sum=11sum=-10sum=12sum=-11sum=13sum=-12sum=14sum=-13sum=15sum=-14sum=16sum=-15sum=17sum=-16sum=18sum=-