//隐藏条件就是1年1月1日是星期一，把这个时间点设为锚点
//计算输入的日期与锚点之间隔了多少天
//天数对7取余，所得结果就是星期几
#include <cstdio>
#include <cstring>

int month[13][2]={
    {0,0},{31,31},{28,29},{31,31},{30,30},{31,31},{30,30},{31,31},{31,31},
    {30,30},{31,31},{30,30},{31,31}
};
char month_name[13][20]={
    "","January","February","March","April","May","June","July","August",
    "September","October","November","December"
};
char week_name[7][20]={
    "Sunday","Monday","Tuesday","Wednesday","Thursday","Friday","Saturday"
};
bool isLeap(int year){
    return (year%4==0&&year%100!=0)||(year%400==0);
}
int main(){
    int d,m,y;
    char s[20];
    while(scanf("%d%s%d",&d,s,&y)!=EOF){
        for(m=1;m<=12;m++){
            if(strcmp(s,month_name[m])==0) break;
        }
        int y1=1,m1=1,d1=1,day=1;
        while(y1<y||m1<m||d1<d){
            d1++;
            if(d1==month[m1][isLeap(y1)]+1){
                d1=1;
                m1++;
            }
            if(m1==13){
                m1=1;
                y1++;
            }
            day++;
        }
        printf("%s\n",week_name[day%7]);
    }
    return 0;
}

Python四行代码，利用datetime库。

import datetime
while True:
    try:
        month = ['January', 'February', 'March', 'April', 'May', 'June', 'July', 'August', 'September', 'October',
                 'November', 'December']
        week = ["Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"]
        d, m, y = input().split()
        print(week[int(datetime.datetime(int(y), month.index(m) + 1, int(d)).strftime("%w"))])
    except:
        break

看了下面的答案后，发现还可以更简单，"A"直接输出的是Tuesday这种格式，太方便了。于是代码可以缩成三行：

import datetime
while True:
    try:
        month = ['January', 'February', 'March', 'April', 'May', 'June', 'July', 'August', 'September', 'October','November', 'December']
        d, m, y = input().split()
        print((datetime.datetime(int(y), month.index(m) + 1, int(d)).strftime("%A")))
    except:
        break

人生苦短，我爱python。

#include <cstdio>
#include <cstring>
#include <algorithm>
#include <iostream>
#include <vector>
#include <map>
#include <cmath>
#include <set>
#include <queue>
 
using namespace std;
map<string, int> mp;
map<int,string> _mp;
string months[] = {"","January","February","March","April","May","June","July","August","September","October","November","December"};
string weekdays[] = {"","Monday","Tuesday","Wednesday","Thursday","Friday","Saturday","Sunday"};
void init()
{
    for(int i = 1;i<=12;i++)
    {
        mp[months[i]] = i;
    }
    for(int i = 1 ;i<=7;i++)
    _mp[i] = weekdays[i]; 
}
int main()
{
    int d,y;
    string m;
    init();
    while(cin >> d >> m >> y)
    {
        int mm = mp[m];
        if(mm == 1 || mm == 2)
        {
            mm += 12;
            y--;
        }
        int ans = (d+2*mm+3*(mm+1)/5+y+y/4-y/100+y/400)%7 + 1;
        cout << _mp[ans] << endl;
    }
}

import datetime
print datetime.datetime.strptime(raw_input(), "%d %B %Y").strftime("%A")

Python其实只要两行233

//王道机试指南解法
#include<iostream>
#include<cstring>
#define ISLEAP(x)  (x%4==0&&x%100!=0)||x%400==0?1:0
int YofM[13][2] = {
    0,0,31,31,28,29,31,31,30,30,31,31,30,30,
    31,31,31,31,30,30,31,31,30,30,31,31
};
struct Data {
    int data;
    int month;
    int year;
    Data() {
        data = 1; month = 0; year = 0;
    }
    void nextdata() {
        data++;
        if (data>YofM[month][ISLEAP(year)]) {
            month++; data = 1;
            if (month>12)
            {
                year++; month = 1;
            }
        }//if
    }
};
char monthName[13][20] = {
    "","January","February","March","April","May","June","July",
    "August","September","October","November","December"
};
char weekName[7][20] = {
    "Sunday","Monday","Tuesday","Wednesday","Thursday","Friday","Saturday"
};
int Daycount[3001][13][32];
int main() {
    Data D;
    int num = 0;
    while (D.year <= 3000) {
        Daycount[D.year][D.month][D.data] = num;
        D.nextdata();
        num++;
    }//while
    int d, m, y;
    char s[20];
    while (std::cin >> d >> s >> y) {
        for (m = 1; m<13; m++) {
            if (strcmp(s, monthName[m]) == 0) { break; }
        }//for
        int i = Daycount[y][m][d] - Daycount[2018][2][25];
        std::cout << weekName[(i % 7 + 7) % 7];
    }//while
    return 0;
}

// java
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.time.DayOfWeek;
import java.time.LocalDate;
import java.time.temporal.ChronoUnit;
import java.util.*;

public class Main {
    public static void main(String[] args) {
        Map<Object, Object> map = new HashMap<>();
        map.put("January", 1);
        map.put("February", 2);
        map.put("March", 3);
        map.put("April", 4);
        map.put("May", 5);
        map.put("June", 6);
        map.put("July", 7);
        map.put("August", 8);
        map.put("September", 9);
        map.put("October", 10);
        map.put("November", 11);
        map.put("December", 12);

        map.put(7, "Sunday");
        map.put(1, "Monday");
        map.put(2, "Tuesday");
        map.put(3, "Wednesday");
        map.put(4, "Thursday");
        map.put(5, "Friday");
        map.put(6, "Saturday");

        Scanner input = new Scanner(System.in);
        String[] split = input.nextLine().split(" ");
        int day = Integer.parseInt(split[0]);
        int month = (int) map.get(split[1]);
        int year = Integer.parseInt(split[2]);
        LocalDate date = LocalDate.of(year, month, day);
        DayOfWeek ofWeek = date.getDayOfWeek();

        System.out.println(map.get(ofWeek.getValue()));
    }
}

// c++
#include<bits/stdc++.h>
using namespace std;
bool isLeapYear(int year) {
    return (year % 400 == 0) || (year % 4 == 0 && year % 100 != 0);
}

int daysOfYear(int year) {
    if(isLeapYear(year)) return 366;
    return 365;
}


int daytab[2][13] = {
    {0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31},
    {0, 31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31}
};

vector<string> months = {"January","February","March","April","May","June","July","August","September","October","November","December"};
vector<string> weekdays = {"Monday","Tuesday","Wednesday","Thursday","Friday","Saturday","Sunday"};

int main() {
    int year, month, day;
    //char monthStr[20];
    string monthStr;
    //scanf("%2d %s %4d", &day, monthStr, &year);
    while( cin >> day >> monthStr >> year) {
        //cout << day << monthStr << year;
        for(int i = 0; i < months.size(); i++) {
            if(monthStr == months[i]) {
               month = i;
               break;
            }
        }
        int y = year;
        long long allDay = 0;
        // 计算从公元1年1月1号到当前的天数
        for(int i = 1; i < year; i++) {
            allDay += daysOfYear(i);
        }
        for(int i = 0; i <= month; i++) {
            allDay += daytab[isLeapYear(year)][i];
        }
        allDay += day - 1;
        cout << weekdays[allDay % 7] << endl;
    }
}

#include<iostream>
#include<stdio.h>
#include<string.h>
using namespace std;
char  mon[][20]={" ","January","February","March","April","May","June","July","August","September","October","November","December"};
char  week[][20]={"Sunday","Monday","Tuesday","Wednesday","Thursday","Friday","Saturday"};
int day[13]={0,31,28,31,30,31,30,31,31,30,31,30,31};
bool leap(int y){
  if(y%400==0||(y%4==0&&y%100!=0))return true;
  else return false;
}
int days(int y,int m,int d){
  int sum=0;
  for(int i=1;i<y;++i){
    if(leap(i))sum+=366;
    else sum+=365;
    }
  if(leap(y))day[2]=29;
  else day[2]=28;
  for(int j=1;j<m;++j){
    sum+=day[j];
  }
  sum+=d;
  return sum;
}
int main(){
  int y,m,d;
  char month[20];

  while(cin>>d>>month>>y){
    for(int i=1;i<13;++i){

        if(strcmp(month,mon[i])==0){
           m=i;
           break;
        }
    }
    int cnt1=days(y,m,d);
    int cnt2=days(2018,3,7);
    int del=cnt1-cnt2;
    del=del%7;
    cout<<week[(del+3)%7];
  }
  return 0;
}

	

#include<stdio.h>
#include<string.h>
#define ISYEAR(x) x%100!=0&&x%4==0||x%400==0?1:0
int dateOfMonth[13][2]={
    0,0,
    31,31,
    28,29,
    31,31,
    30,30,
    31,31,
    30,30,
    31,31,
    31,31,
    30,30,
    31,31,
    30,30,
    31,31
};
struct Date{
    int Year;
    int Month;
    int Day;
    void NextDay(){
        Day++;
        if(Day>dateOfMonth[Month][ISYEAR(Year)]){
            Day=1;
            Month++;
            if(Month>12){
                Month=1;
                Year++;
            }
        }
    }
};
int buf[3001][13][32];
char MonthName[13][20]={
    " ",
    "January",
    "February",
    "March",
    "April",
    "May",
    "June",
    "July",
    "August",
    "September",
    "October",
    "November",
    "December"
};
char WeekName[7][20]={
    "Sunday", 
    "Monday",
    "Tuesday",
    "Wednesday",
    "Thursday",
    "Friday",
    "Saturday"
};
int main(){
    Date tmp;
    tmp.Year=0;
    tmp.Month=1;
    tmp.Day=1;
    int cnt=0;
    while(tmp.Year!=3001){
        buf[tmp.Year][tmp.Month][tmp.Day]=cnt;
        tmp.NextDay();
        cnt++;
    }
    int d,m,y;
    char s[20];
    while(scanf("%d%s%d",&d,s,&y)!=EOF){
        for(m=1;m<13;m++){
            if(strcmp(MonthName[m],s)==0)
                break;
        }
    }
    int days=(buf[y][m][d]-buf[2012][7][16])+1;
    printf("%s",WeekName[(days%7+7)%7]);    
}

from datetime import datetime
Month = {'May': 5, 'December': 12, 'October': 10, 'August': 8,
         'July': 7, 'February': 2, 'January': 1, 'September': 9,
         'March': 3, 'April': 4, 'November': 11, 'June': 6}
try:
    while 1:
        k = raw_input().split()
        print datetime(int(k[2]),Month[k[1]],int(k[0])).strftime('%A').capitalize()
except:
    pass

有一个隐藏条件就是1年1月1日为星期一

#include<iostream>
#include<string>

using namespace std;

int date[2][12] = {
    {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31},
    {31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31}
};

int getMonth(string s){
    if(s == "January") return 1;
    else if(s == "February") return 2;
    else if(s == "March") return 3;
    else if(s == "April") return 4;
    else if(s == "May") return 5;
    else if(s == "June") return 6;
    else if(s == "July") return 7;
    else if(s == "August") return 8;
    else if(s == "September") return 9;
    else if(s == "October") return 10;
    else if(s == "November") return 11;
    else if(s == "December") return 12;
    else return 0;
}

string getWeek(int n){
    if(n == 0) return "Monday";
    else if(n == 1) return "Tuesday";
    else if(n == 2) return "Wednesday";
    else if(n == 3) return "Thursday";
    else if(n == 4) return "Friday";
    else if(n == 5) return "Saturday";
    else if(n == 6) return "Sunday";
    else return "";
}

int over(int &m, int &d, int &yes){
    int over = d;
    for(int i = 0;i < m - 1;i++) over += date[yes][i];
    return over;
}

int isLeap(int &y){
    int x = y;
    if(x % 100 == 0) x /= 100;
    return x % 4 == 0;
}

char ch[10];
int day, month, year;
int yes;

int main(){
    while(scanf("%d %s %d", &day, ch, &year) != EOF){
        month = getMonth(ch);
        yes = isLeap(year);
        int left = 0;
        left += over(month, day, yes);
        for(int i = 1;i < year;i++){
            if(isLeap(i)) left += 366;
            else left += 365;
        }
        cout<<getWeek((left - 1) % 7)<<endl;
    }
    return 0;
}

#include <iostream>
#include <stdio.h>
#define leap(x) (x%400 == 0 || (x%4 == 0 && x%100 != 0)?1:0)
#include <string>
#include <string.h>
#include <math.h>

using namespace std;

const int yd[2][13] = 
{
    {0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31},
    {0, 31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31}   
};

char monthName[13][20] = 
{
    "","January","February","March","April","May","June","July",
    "August","September","October","November","December"
};
char weekName[7][20] = 
{
    "Sunday","Monday","Tuesday","Wednesday","Thursday","Friday","Saturday"
};

int days(int year, int month, int date)
{
    int sum = 0;
    for(int i = 1; i < year; i++)
    {
        if(leap(i))
            sum+=366;
        else
            sum+=365;
    }

    for(int i = 1; i < month; i++)
    {
        sum+=yd[leap(year)][i];
    }
    sum+=date;
    return sum;
}

int main()
{
    char Mon[20];
    int date, month, year;
    int remainder;
    while(scanf("%d%s%d", &date, &Mon, &year) != EOF)
    {
        for(month = 1; month < 13; month++)
        {
            if(strcmp(Mon, monthName[month]) == 0)
                break;
        }
        int DayNumber = days(year, month, date);
        int DayNumber1 = days(1, 1 , 1);
        if(DayNumber > DayNumber1)
            remainder = (DayNumber - DayNumber1 + 1) % 7;
        else 
            remainder = (DayNumber1 - DayNumber + 1) % 7;
        cout << weekName[remainder] << endl;
    }
    
    return 0;
}

#include <iostream>
#include <string>
#include <ctime>
#include <stdio.h>
using namespace std;

string output[7]={"Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"};

int m_days[2][12]={31,28,31,30,31,30,31,31,30,31,30,31
				  ,31,29,31,30,31,30,31,31,30,31,30,31}; 
string months[12] = {"January", "February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December"};

void judge(int d,string month, int y){
	int m=0;
	for(int i=0;i<12;i++){
		if(month == months[i]){
			m = i+1;
			break;
		}
	}
	
	//cout<<m<<endl;
	
	if(m==1){
		m = 13;
		y--;
	}
	if(m==2){
		m = 14;
		y--;
	}
	
	int W= (d+2*m+3*(m+1)/5+y+y/4-y/100+y/400+1)%7;
	
	cout<<output[W]<<endl;	
}

int main(){
	ios::sync_with_stdio(false);
		
	int d,y;
	string month;

	while(cin>>d){
		
		cin>>month;
		cin>>y;
		judge(d,month,y);
	}
	
	
	return 0;
}

#include <iostream>
#include <cstdio>
#include <cstring>

using namespace std;

const int dayTab[2][13] = {
    {0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31},
    {0, 31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31}
};
const int yearTab[2] = {365, 366};
const string monthNameTab[13] = {
    "NULL", "January", "February", "March", "April", "May", "June",
    "July", "August", "September", "October", "November", "December"
};
const string weekTab[7] = {
    "Monday", "Tuesday", "Wednesday", 
    "Thursday", "Friday", "Saturday", "Sunday"
};

int isLeapYear(int year);

//公元1年1月1日为星期一
int main() {
    int year, month, day, number, start;
    string monthName;
    while (cin >> day >> monthName >> year) {
        number = -1;
        start = 1;
        while (start != year) {
            number += yearTab[isLeapYear(start)];
            start++;
        }
        for (int i = 0; i < 13; ++i) {
            if (monthName != monthNameTab[i]) {
                number += dayTab[isLeapYear(year)][i];
            } else {
                break;
            }
        }
        number += day;    // 从公元1年1月1日累加至给定日期的天数
        cout << weekTab[number % 7] << endl;
    }
}

int isLeapYear(int year) {
    if (year % 400 == 0 || (year % 4 == 0 && year % 100 != 0)) {
        return 1;
    }
    return 0;
}

#include<iostream>
#include <map>

using namespace std;
string monthStr[]{"January", "February", "March", "April", "May", "June", "July",
                  "August", "September", "October", "November", "December"};
string week[]{"Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"};
map<string, int> mm;

int main() {
    int day, month, year;
    string mon;
    for (int x = 1; x <= 12; x++) mm[monthStr[x - 1]] = x;
    while (cin >> day >> mon >> year) {
        month = mm[mon];
        if (month == 1 || month == 2) {
            month += 12;
            year--;
        }
        int w = (day + 2 * month + 3 * (month + 1) / 5 + year + year / 4 - year / 100 + year / 400 + 1) % 7;
        cout << week[w] << endl;
    }
}