思路是flag改变符号

#define _CRT_SECURE_NO_WARNINGS 1
#include<stdio.h>
int main()
{
double sum = 0.0;
int i = 0;
int flag = 1;
for (i = 1; i <= 100; i++)
{

sum += flag*1.0 / i;
flag = -flag;
}
printf("%f\n", sum);
}