#include <stdio.h>
int main(){
	int year,month,day;
	scanf("%d %d",&year,&month);
	switch(month){
		case 1:
			day = 31;
		case 2:
			day = (year%100?year%400:year%4)?29:28;			
	}
	printf("%d",day);
}

输入 19 1 时，会输出29，请问这是怎么回事啊？

C:\Users\admin\Documents\c2.cpp	In function 'int main()':
7	8	C:\Users\admin\Documents\c2.cpp	[Warning] this statement may fall through [-Wimplicit-fallthrough=]
8	3	C:\Users\admin\Documents\c2.cpp	[Note] here