题解 | #Day of Week#

#include<stdio.h>
#include<stdlib.h>
#include<string.h>

bool IsLeapYear(int year){
	return year%100!=0&&year%4==0||year%400==0;
}
int DaysInMonth(int year,int month){
	switch(month){
		case 1:  return 31;
		case 2:  return IsLeapYear(year)?29:28;
		case 3:  return 31;
		case 4:  return 30;
		case 5:  return 31;
		case 6:  return 30;
		case 7:  return 31;
		case 8:  return 31;
		case 9:  return 30;
		case 10: return 31;
		case 11: return 30;
		case 12: return 31; 
        default:
            return 0;
	}
}

int getDeltaInYears(int from,int to){
	int current = from+1;
	int acc = 0;
	for(;current<to;current++){
		acc += IsLeapYear(current)?366:365;
	}
	return acc;
}
int getDeltaInYear(int year,int monthFrom,int dateFrom,int monthTo,int dateTo){
	if(monthFrom==monthTo) return dateTo-dateFrom;
	int acc = DaysInMonth(year,monthFrom)-dateFrom+1;
	int currentMonth = monthFrom+1;
	for(;currentMonth<monthTo;currentMonth++){
		acc+= DaysInMonth(year,currentMonth);	
	}
	acc += (dateTo-1);
	return acc;
}

char monthNameList[13][10] = {
	"None",
	"January",
	"February",
	"March",
	"April",
	"May",
	"June",
	"July",
	"August",
	"September",
	"October",
	"November",
	"December"
};
int getMonthByName(char* monthName){//字符串比较用strcmp 
	int month=1;
	for(;month<=12;month++){
		if(strcmp(monthName,monthNameList[month])==0) break;
		//strcmp是==0时相等，容易记错 
	}
	return month;
}
char dayName[7][10]= {
	"Sunday",
	"Monday",
	"Tuesday",
	"Wednesday",
	"Thursday",
	"Friday",
	"Saturday"
};
void Question2_7(){//习题2.7 上海交大 计算给定日期是周几 
	int date,month,year;
	char* monthName = (char*)malloc(sizeof(char*));
	while(scanf("%d %s %d",&date,monthName,&year)!=EOF){//char*的输入输出不用& 
		month = getMonthByName(monthName); 
	
		int acc=0;
		if(year==2001){//2001.1.1是周一 
			acc = getDeltaInYear(2011,1,1,month,date);
		} else {
			if(year>2001){//工具函数是几道题通用的，那边自动+1了，这里要-1对齐
				acc+= getDeltaInYears(2000,year);
				acc+= getDeltaInYear(year,1,1,month,date);
			} else {
				acc+= getDeltaInYear(year,month,date,12,31);
				acc+= 1;
				acc+= getDeltaInYears(year,2001);
			}
		}
		int mod = acc % 7;
		printf("%s\n",dayName[(mod+1)%7]);
	}
}

int main(){
	
	Question2_7();
	
	return 0;
}