用sign控制每轮循环中i的符号变化
牛牛学数列
http://www.nowcoder.com/questionTerminal/0b97367cd2184c12a0e02f7c223aee11
#include<stdio.h>
main()
{int n,i,sign=1,sum=0;
scanf("%d",&n);
for(i=1;i<=n;i++)
{sum=sum+sign*i;
sign*=-1;}
printf("%d",sum);}
牛牛学数列
http://www.nowcoder.com/questionTerminal/0b97367cd2184c12a0e02f7c223aee11
main()
{int n,i,sign=1,sum=0;
scanf("%d",&n);
for(i=1;i<=n;i++)
{sum=sum+sign*i;
sign*=-1;}
printf("%d",sum);}
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