5.5

#include<stdio.h>int main(){    int year,flag;    printf("Enter year:");    scanf("%d",&year);    if(year%4==0&&year%100!=0||year%400==0)        flag=1;    else        flag=0;    if(flag==1)        printf("%d is a leap year!\n",year);    else        printf("%d is not a leap year\n",year);    return 0;}

测试结果：