{
flag=-flag;
sum=sum-i*flag;
}
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flag=-flag的时候，也就是第一次循环sum那栏是sum+i，也就是1，第二次循环的时候用来循环的是第一次变化过的flag，也就是这时候用来循环的是-flag，而在第二次循环它又一次进行了*-1的操作，变成了正的flag，这时候sun那栏就成了sum-i*flag，也就是成了-2。然后进入第三次循环了。所以他完成了一负一正的运行