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Formatted question description: https://leetcode.ca/all/1360.html
1360. Number of Days Between Two Dates (Easy)
Write a program to count the number of days between two dates.
The two dates are given as strings, their format is YYYYMMDD
as shown in the examples.
Example 1:
Input: date1 = "20190629", date2 = "20190630" Output: 1
Example 2:
Input: date1 = "20200115", date2 = "20191231" Output: 15
Constraints:
 The given dates are valid dates between the years
1971
and2100
.
Solution 1.
When m=1
or m=2
(January or February), we let m=13
or m=14
and let y
decreased by 1. Imagine it is 13th or 14th month of the last year. By doing that, we let the magical formula also work for those two months. (153 * m + 8) // 5
is just a carefully designed way to record the days of each month. More specifically, it is designed to record the difference of days between two months. Suppose we have March 1st and April 1st, (153 * 3 + 8) // 5 = 93
while (153 * 4 + 8) // 5 = 124
, the difference is 31 which is the number of days in March. Suppose we have April 1st to May 1st, (153 * 4 + 8) // 5 = 124
and (153 * 5 + 8) // 5 = 154
, the difference is now 30 which is the number of days in April. You can also check other months.

class Solution { public int daysBetweenDates(String date1, String date2) { String[] array1 = date1.split(""); String[] array2 = date2.split(""); int year1 = Integer.parseInt(array1[0]), month1 = Integer.parseInt(array1[1]), day1 = Integer.parseInt(array1[2]); int year2 = Integer.parseInt(array2[0]), month2 = Integer.parseInt(array2[1]), day2 = Integer.parseInt(array2[2]); if (year1 > year2  year1 == year2 && month1 > month2  year1 == year2 && month1 == month2 && day1 > day2) { int temp = year1; year1 = year2; year2 = temp; temp = month1; month1 = month2; month2 = temp; temp = day1; day1 = day2; day2 = temp; } int numberOfDays1 = numberOfDays(year1, month1, day1); int numberOfDays2 = numberOfDays(year2, month2, day2); if (year1 == year2) return numberOfDays2  numberOfDays1; else { int totalDays = (year2  year1) * 365  numberOfDays1 + numberOfDays2; int checkLeapYear = (int) Math.ceil(year1 / 4.0) * 4; while (checkLeapYear < year2) { if (isLeap(checkLeapYear)) totalDays++; checkLeapYear += 4; } return totalDays; } } public int numberOfDays(int year, int month, int day) { int[] monthDays = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31}; if (isLeap(year)) monthDays[1]++; int numberOfDays = 0; for (int i = month  2; i >= 0; i) numberOfDays += monthDays[i]; numberOfDays += day; return numberOfDays; } public boolean isLeap(int year) { return year % 4 == 0 && year % 100 != 0  year % 400 == 0; } }

// OJ: https://leetcode.com/problems/numberofdaysbetweentwodates/ // Time: O(1) // Space: O(1) // Ref: https://leetcode.com/problems/numberofdaysbetweentwodates/discuss/517605/Similartodayoftheyear class Solution { const int days[12] = { 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 }; bool isLeap(int year) { return year % 4 == 0 && (year % 100 != 0  year % 400 == 0); } int daysFrom1971(string date) { int y = stoi(date.substr(0, 4)), m = stoi(date.substr(5, 2)), d = stoi(date.substr(8)); for (int yr = 1971; yr < y; ++yr) d += isLeap(yr) ? 366 : 365; return d + (m > 2 && isLeap(y)) + accumulate(begin(days), begin(days) + m  1, 0); } public: int daysBetweenDates(string date1, string date2) { return abs(daysFrom1971(date1)  daysFrom1971(date2)); } }; // OJ: https://leetcode.com/problems/numberofdaysbetweentwodates/ // Time: O(1) // Space: O(1) // Ref: https://leetcode.com/problems/numberofdaysbetweentwodates/discuss/517582/PythonMagicalFormula class Solution { int days(string date) { int y = stoi(date.substr(0, 4)), m = stoi(date.substr(5, 2)), d = stoi(date.substr(8)); if (m < 3) { m += 12; y = 1; } return 365 * y + y / 4 + y / 400  y / 100 + d + (153 * m + 8) / 5; } public: int daysBetweenDates(string date1, string date2) { return abs(days(date1)  days(date2)); } };