typedef struct date{

              int year;

              int month;

              int day;

}Date;

void days(int year,int month,int day)

{

        int sum=0;

        int Feb=0;

        if (year%400==0) {

        Feb=29;

        }else if ((year%4==0 )&& (year%100!=0)){

        Feb=29;

        }else{

        Feb=28;

    }

     switch (month) {

          case1:sum=day;

          break;

          case2:sum=31+day;

          break;

          case3:sum=31+Feb+day;

          break;

          case4:sum=31+31+Feb+day;

          break;

          case5:sum=31+31+Feb+30+day;

          break;

          case6:sum=31+31+Feb+30+31+day;

          break;

          case7:sum=31+31+Feb+30+31+30+day;

          break;

          case8:sum=31+31+Feb+30+31+30+31+day;

          break;

          case9:sum=31+31+Feb+30+31+30+31+31+day;

          break;

          case10:sum=31+31+Feb+30+31+30+31+31+30+day;

          break;

          case11:sum=31+31+Feb+30+31+30+31+31+30+31+day;

          break;

          case12:sum=31+31+Feb+30+31+30+31+31+30+31+30+day;

          break;

          default:

          break;

  }

     printf("为本年的第%d天",sum);

}

int main()

{

    Date a;

    printf("please input year:");

    scanf("%d",&a.year);

    printf("please input month:");

    scanf("%d",&a.month);

    printf("please input day:");

    scanf("%d",&a.day);

    days(a.year,a.month,a.day);

    return0;

}

算法写的水平欠佳但运行结果还算正确