1185. Day of the Week 一周中的第几天

@TOC

# 题目描述

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"
``````

Constraints:

• The given dates are valid dates between the years 1971 and 2100.

# 解题方法

# 计算与1971-1-1之间天数

C++代码如下：

``````class Solution {
public:
string dayOfTheWeek(int day, int month, int year) {
int days = countDaysFrom1971(day, month, year);
return weekName[(days+ 5) % 7];
}
bool isLeapYear(int year) {
return (year % 4 == 0 && year % 100 != 0) || year % 400 == 0;
}
int countDaysFrom1971(int day, int month, int year) {
int res = 0;
bool leapYear = isLeapYear(year);
year --;
while (year >= 1971) {
res += isLeapYear(year) ? 366 : 365;
year--;
}
while (month > 1) {
res += daysOfMonth[leapYear][month - 2];
month--;
}
res += day - 1;
return res;
}
int daysOfMonth[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 weekName[7] = {"Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"};
};
``````

# 日期

2019 年 9 月 18 日 —— 今日又是九一八