#include <stdio.h>
void main()
{
    int day,month,year,sum,leap;
    printf("\nplease input year,month,day\n");
    scanf("%d%d%d",&year,&month,&day);
    switch(month)
    {
    case 1:sum=0;break;
    case 2:sum=31;break;
    case 3:sum=59;break;
    case 4:sum=90;break;
    case 5:sum=120;break;
    case 6:sum=151;break;
    case 7:sum=181;break;
    case 8:sum=212;break;
    case 9:sum=243;break;
    case 10:sum=273;break;
    case 11:sum=304;break;
    case 12:sum=334;break;
    default:printf("data error");break;
    }
    sum=sum+day;
    if(year%400==0||(year%4==0&&year%100!=0))
        leap=1;
    else
        leap=0;
    if(leap==1&&month>2)
        sum++;
    printf("It is the %dth day .\n");
}

倒数第二行换成
printf("It is the %dth day",sum);
楼主漏掉东西了。