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);
}