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和日期相关的代码和bug——一道力扣题中的小发现

2023-03-13

int year day
目录DayoftheWeek题目大意常规方法Python代码Golang代码 C++代码基姆拉尔森公式Python代码Golang代码C++代码使用库函数Python代码Golang代码C++代码DayoftheWeekGivenadate,returnthecorrespondingda

目录

Day of the Week

题目大意

常规方法

Python代码

Golang代码

 C++代码

基姆拉尔森公式

Python代码

Golang代码

C++代码

使用库函数

Python代码

Golang代码

C++代码

Day of the Week

Given a date, return the corresponding day of the week for that date.
The input is given as three integers representing the day , month and year respectively.
Return the answer as one of the following values {"Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"} .

Example 1:
Input: day = 31, month = 8, year = 2019
Output: "Saturday"
Example 2:
Input: day = 18, month = 7, year = 1999
Output: "Sunday"
Example 3:
Input: day = 15, month = 8, year = 1993
Output: "Sunday"

题目大意

给你一个日期，请你设计一个算法来判断它是对应一周中的哪一天。
输入为三个整数： day、 month 和 year，分别表示日、月、年。
您返回的结果必须是这几个值中的一个 {"Sunday", "Monday", "Tuesday", "Wednesday", "Thursday",
"Friday", "Saturday"}。
提示：
给出的日期一定是在 1971 到 2100 年之间的有效日期。
解题思路：
给出一个日期，要求算出这一天是星期几。

常规方法

从1971.1.1起，先累计整年year、整月month-1的天数，再加上最后一个月month的天数day，然后总天数减1后与7求余。最后得到的余数在星期字串数组中位置索引，显然前提要知道1971.1.1这个基准日期是星期几，再作一个索引位移就是答案。

另外常规方法还需要判断year是否闰年，规则：y%4==0 and y%100!=0 or y%400==0，据说是1582

Python代码

python代码非常简单，不需另外导入库只用内置函数就能搞定。

  1. class Solution(object):
  2.     def DayOfWeek(self, year, month, day):
  3.         days = 0
  4.         isLeapYear = lambda y:y%4==0 and y%100!=0 or y%400==0
  5.         monthday = [31,28,31,30,31,30,31,31,30,31,30,31]
  6.         week = ["Sunday","Monday","Tuesday","Wednesday","Thursday","Friday","Saturday"]
  7.         monthday[1] = 29 if isLeapYear(year) else 28
  8.         for i in range(1971,year):
  9.             days += 366 if isLeapYear(i) else 365
  10.         days += sum(monthday[:month-1], day-1)
  11.         return week[(days+5)%7]
  12.  
  13. if __name__ == "__main__":
  14.     s = Solution()
  15.     print(s.DayOfWeek(2019,8,31))
  16.     print(s.DayOfWeek(1999,7,18))
  17.     print(s.DayOfWeek(1993,8,15))
  19.     print(s.DayOfWeek(1971,6,12))
  20.     print(s.DayOfWeek(2023,2,22))
  21.     print(s.DayOfWeek(2040,6,13))

输出：

Saturday
Sunday
Sunday
Saturday
Wednesday
Wednesday

Golang代码

基本原理相同，另外自定义一个数组求和公式即可。

  1. package main
  2.  
  3. import "fmt"
  4.  
  5. func DayOfWeek(year int, month int, day int) string {
  6. days := 0
  7. isLeapYear := func(y int) bool {
  8. return y%4 == 0 && y%100 != 0 || y%400 == 0
  9. }
  10. Sum := func(nums []int, initNum int) int {
  11. var sumNum int = 0
  12. for _, num := range nums {
  13. sumNum += num
  14. }
  15. return sumNum + initNum
  16. }
  17. monthday := []int{31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31}
  18. week := []string{"Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"}
  19. if isLeapYear(year) {
  20. monthday[1] = 29
  21. } else {
  22. monthday[1] = 28
  23. }
  24. for i := 1971; i < year; i++ {
  25. if isLeapYear(i) {
  26. days += 366
  27. } else {
  28. days += 365
  29. }
  30. }
  31. days += Sum(monthday[:month-1], day-1)
  32. return week[(days+5)%7]
  33. }
  34.  
  35. func main() {
  36.  
  37. fmt.Println(DayOfWeek(2019, 8, 31))
  38. fmt.Println(DayOfWeek(1999, 7, 18))
  39. fmt.Println(DayOfWeek(1993, 8, 15))
  40.  
  41. fmt.Println(DayOfWeek(1971, 6, 12))
  42. fmt.Println(DayOfWeek(2023, 2, 22))
  43. fmt.Println(DayOfWeek(2040, 6, 13))
  44.  
  45. }

输出：

Saturday

Sunday

Sunday

Saturday

Wednesday

Wednesday

成功: 进程退出代码 0.

 C++代码

引入C++11的容器vector，可以省掉最后一个非整年的各月份日数循环累加，只要用<numeric>库中的函数accumulate，方便累加非整年的各月份日数，并且把day作为基准数一并累加掉。

  1. #include<iostream>
  2. #include<vector>
  3. #include<numeric>
  4. using namespace std;
  5.  
  6. class Solution
  7. {
  8. public:
  9.     string DayOfWeek(int year, int month, int day)
  10.     {
  11. int days = 0;
  12. auto isLeapYear = [](int y) { return y%4==0 && y%100!=0 || y%400==0; };
  13.         vector<int> monthday = {31,28,31,30,31,30,31,31,30,31,30,31};
  14.         vector<string> week = {"Sunday","Monday","Tuesday","Wednesday","Thursday","Friday","Saturday"};
  15. monthday[1] = isLeapYear(year) ? 29 : 28;
  16. for (int i=1971;i<year;i++)
  17. days += isLeapYear(i) ? 366 : 365;
  18. days += accumulate(monthday.begin(), monthday.begin()+month-1, day-1);
  19.         return week[(days+5)%7];
  20.     }
  21. };
  22.  
  23. int main()
  24. {
  25. Solution s;
  27. cout << s.DayOfWeek(2019,8,31) << endl;
  28. cout << s.DayOfWeek(1999,7,18) << endl;
  29. cout << s.DayOfWeek(1993,8,15) << endl;
  31. cout << s.DayOfWeek(1971,6,12) << endl;
  32. cout << s.DayOfWeek(2023,2,22) << endl;
  33. cout << s.DayOfWeek(2040,6,13) << endl;
  35. return 0;
  36. }

Dev C++ 6.3 编译通过：

Saturday
Sunday
Sunday
Saturday
Wednesday
Wednesday

--------------------------------
Process exited after 0.02175 seconds with return value 0
请按任意键继续. . .

 

基姆拉尔森公式

万能的日期计算公式，不用知道基准日是哪一天，也不需要判断year是否为闰年。

公式：weekday = (day+2month+3(month+1)/5+year+year/4-year/100+year/400+1)%7

注意：1月和2月需看做上一年的13月与14月，即 month<3时， year-=1; month+=12

Python代码

  1. class Solution(object):
  2.     def DayOfWeek(self, year, month, day):
  3.         week = ["Sunday","Monday","Tuesday","Wednesday","Thursday","Friday","Saturday"]
  4.         if month<3: year, month = year-1, month+12
  5.         weekday = (day+2*month+3*(month+1)//5+year+year//4-year//100+year//400+1)%7
  6.         return dict(zip(range(7),week)).get(weekday)
  7.  
  8. if __name__ == "__main__":
  9.     s = Solution()
  10.     print(s.DayOfWeek(2019,8,31))
  11.     print(s.DayOfWeek(1999,7,18))
  12.     print(s.DayOfWeek(1993,8,15))
  14.     print(s.DayOfWeek(1971,6,12))
  15.     print(s.DayOfWeek(2023,2,22))
  16.     print(s.DayOfWeek(2040,6,13))

Golang代码

  1. package main
  2.  
  3. import "fmt"
  4.  
  5. func DayOfWeek(year int, month int, day int) string {
  6. week := []string{"Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"}
  7. if month < 3 {
  8. year -= 1
  9. month += 12
  10. }
  11. weekday := (day + 2*month + 3*(month+1)/5 + year + year/4 - year/100 + year/400 + 1) % 7
  12. return week[weekday]
  13. }
  14.  
  15. func main() {
  16.  
  17. fmt.Println(DayOfWeek(2019, 8, 31))
  18. fmt.Println(DayOfWeek(1999, 7, 18))
  19. fmt.Println(DayOfWeek(1993, 8, 15))
  20.  
  21. fmt.Println(DayOfWeek(1971, 6, 12))
  22. fmt.Println(DayOfWeek(2023, 2, 22))
  23. fmt.Println(DayOfWeek(2040, 6, 13))
  24.  
  25. }

C++代码

  1. #include<iostream>
  2. using namespace std;
  3.  
  4. class Solution
  5. {
  6. public:
  7.     string DayOfWeek(int year, int month, int day)
  8.     {
  9.         const char *week[7] = {"Sunday","Monday","Tuesday","Wednesday","Thursday","Friday","Saturday"};
  10. if (month < 3) {
  11.         year -= 1;
  12.         month += 12;
  13.     } 
  14.     int weekday = (day+2*month+3*(month+1)/5+year+year/4-year/100+year/400+1)%7;
  15.     return week[weekday];
  16.     }
  17. };
  18.  
  19. int main()
  20. {
  21. Solution s;
  23. cout << s.DayOfWeek(2019,8,31) << endl;
  24. cout << s.DayOfWeek(1999,7,18) << endl;
  25. cout << s.DayOfWeek(1993,8,15) << endl;
  27. cout << s.DayOfWeek(1971,6,12) << endl;
  28. cout << s.DayOfWeek(2023,2,22) << endl;
  29. cout << s.DayOfWeek(2040,6,13) << endl;
  31. return 0;
  32. }

 

使用库函数

Python代码

datetime库

  1. import datetime
  2.  
  3. class Solution(object):
  4.     def DayOfWeek(self, year, month, day):
  5.         week = ["Sunday","Monday","Tuesday","Wednesday","Thursday","Friday","Saturday"]
  6.         weekday = datetime.date(year,month,day).isoweekday()
  7.         return week[weekday%7]
  8.  
  9. if __name__ == "__main__":
  10.     s = Solution()
  11.     print(s.DayOfWeek(2019,8,31))
  12.     print(s.DayOfWeek(1999,7,18))
  13.     print(s.DayOfWeek(1993,8,15))
  15.     print(s.DayOfWeek(1971,6,12))
  16.     print(s.DayOfWeek(2023,2,22))
  17.     print(s.DayOfWeek(2040,6,13))

calendar库 

  1. import calendar
  2.  
  3. class Solution(object):
  4.     def DayOfWeek(self, year, month, day):
  5.         week = ["Sunday","Monday","Tuesday","Wednesday","Thursday","Friday","Saturday"]
  6.         weekday = calendar.weekday(year,month,day)+1
  7.         return week[weekday%7]
  8.  
  9. if __name__ == "__main__":
  10.     s = Solution()
  11.     print(s.DayOfWeek(2019,8,31))
  12.     print(s.DayOfWeek(1999,7,18))
  13.     print(s.DayOfWeek(1993,8,15))
  15.     print(s.DayOfWeek(1971,6,12))
  16.     print(s.DayOfWeek(2023,2,22))
  17.     print(s.DayOfWeek(2040,6,13))

Golang代码

time库，超级省事，连星期数组都不用了。

  1. package main
  2.  
  3. import (
  4. "fmt"
  5. "time"
  6. )
  7.  
  8. func DayOfWeek(year int, month int, day int) string {
  9. return time.Date(year, time.Month(month), day, 0, 0, 0, 0, time.Local).Weekday().String()
  10. }
  11.  
  12. func main() {
  13.  
  14. fmt.Println(DayOfWeek(2019, 8, 31))
  15. fmt.Println(DayOfWeek(1999, 7, 18))
  16. fmt.Println(DayOfWeek(1993, 8, 15))
  17.  
  18. fmt.Println(DayOfWeek(1971, 6, 12))
  19. fmt.Println(DayOfWeek(2023, 2, 22))
  20. fmt.Println(DayOfWeek(2040, 6, 13))
  21.  
  22. }

C++代码

ctime库

  1. #include<iostream>
  2. #include<ctime>
  3. using namespace std;
  4.  
  5. class Solution
  6. {
  7. public:
  8.     string DayOfWeek(int year, int month, int day)
  9.     {
  10.         const char *week[7] = {"Sunday","Monday","Tuesday","Wednesday","Thursday","Friday","Saturday"};
  11.     struct tm t = {0};
  12.     t.tm_year = year - 1900;
  13.     t.tm_mon = month - 1;
  14.     t.tm_mday = day;
  15.     mktime(&t);
  16.     return week[t.tm_wday%7];
  17.     }
  18. };
  19.  
  20. int main()
  21. {
  22. Solution s;
  24. cout << s.DayOfWeek(2019,8,31) << endl;
  25. cout << s.DayOfWeek(1999,7,18) << endl;
  26. cout << s.DayOfWeek(1993,8,15) << endl;
  28. cout << s.DayOfWeek(1971,6,12) << endl;
  29. cout << s.DayOfWeek(2023,2,22) << endl;
  30. cout << s.DayOfWeek(2040,6,13) << endl;
  31.  
  32. return 0;
  33. }

输出：

Saturday
Sunday
Sunday
Saturday
Wednesday
Sunday

--------------------------------
Process exited after 0.02402 seconds with return value 0
请按任意键继续. . .

发现没？ 2040.6.13返回的星期是错的！

网上查了资料，原来ctime库的CTime对象是有指定范围的：

static CTime WINAPI GetCurrentTime( );
获取系统当前日期和时间。

返回表示当前日期和时间的CTime对象。
int GetYear( ) const；
获取CTime对象表示时间的年份。

范围从1970年1月1日到2038年1月18日。

时间范围测试：

  1. #include<iostream>
  2. #include<ctime>
  3. using namespace std;
  4.  
  5. class Solution
  6. {
  7. public:
  8.     string DayOfWeek(int year, int month, int day)
  9.     {
  10.         const char *week[7] = {"Sunday","Monday","Tuesday","Wednesday","Thursday","Friday","Saturday"};
  11.     struct tm t = {0};
  12.     t.tm_year = year - 1900;
  13.     t.tm_mon = month - 1;
  14.     t.tm_mday = day;
  15.     mktime(&t);
  16.     return week[t.tm_wday%7];
  17.     }
  18. };
  19.  
  20. int main()
  21. {
  22. Solution s;
  23. for (int i=16;i<25;i++)
  24. cout << i << ":" << s.DayOfWeek(2038,1,i) << endl;
  25.  
  26. return 0;
  27. }

测试结果：

16:Saturday
17:Sunday
18:Monday
19:Tuesday
20:Sunday
21:Sunday
22:Sunday
23:Sunday
24:Sunday

--------------------------------
Process exited after 0.05159 seconds with return value 0
请按任意键继续. . .

2038.1.19日的星期也对，之后的全部返回Sunday。

修改这个问题，技术上一点问题都没有。 目前C++都发展到C++20了，而我用的是C++11，暂不知道之后版本的库文件有没有对此问题作过更新。那么，问题来了：

之前用C语言写的的软件，用ctime或者time.h获取时间的软件在2038年1月19日之后都会发生错误。还好，还有整整15年时间来改正这个“时间Bug”。

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