5.5

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

知识点：取余。
心得体会：注意并且和或的符号。