Write a program to count the number of days between two dates.

    The two dates are given as strings, their format is YYYY-MM-DD as shown in the examples.

    Example 1:

    1. Input: date1 = "2019-06-29", date2 = "2019-06-30"
    2. Output: 1

    Example 2:

    1. Input: date1 = "2020-01-15", date2 = "2019-12-31"
    2. Output: 15

    Constraints:

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

    题意

    计算两个日期之间的天数。

    思路

    几种方法：

    1. 直接调用java API；
    2. 累加较小日期的天数，直到两日期相等；
    3. 将每个日期都转化为到1970-01-00的天数，最后相减(实际上就是自己实现了java API)

    代码实现 - API

    1. import java.time.LocalDate;
    3. class Solution {
    4.     public int daysBetweenDates(String date1, String date2) {
    5.         return (int) Math.abs(LocalDate.parse(date1).toEpochDay() - LocalDate.parse(date2).toEpochDay());
    6.     }
    7. }

    代码实现 - 累加

    1. class Solution {
    2.     public int daysBetweenDates(String date1, String date2) {
    3.         if (date1.compareTo(date2) > 0) {
    4.             String temp  = date1;
    5.             date1 = date2;
    6.             date2 = temp;
    7.         }
    9.         String[] s1 = date1.split("-"), s2 = date2.split("-");
    10.         int y1 = Integer.parseInt(s1[0]), y2 = Integer.parseInt(s2[0]);
    11.         int m1 = Integer.parseInt(s1[1]), m2 = Integer.parseInt(s2[1]);
    12.         int d1 = Integer.parseInt(s1[2]), d2 = Integer.parseInt(s2[2]);
    13.         int[] days = {0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
    14.         int count = 0;
    16.         days[2] = isLeapYear(y1) ? 29 : 28;
    18.         while (y1 != y2 || m1 != m2 || d1 != d2) {
    19.             count++;
    20.             d1++;
    21.             if (d1 == days[m1] + 1) {
    22.                 d1 = 1;
    23.                 m1++;
    24.             }
    25.             if (m1 == 13) {
    26.                 m1 = 1;
    27.                 y1++;
    28.                 days[2] = isLeapYear(y1) ? 29 : 28;
    29.             }
    30.         }
    32.         return count;
    33.     }
    35.     private boolean isLeapYear(int year) {
    36.         return year % 100 != 0 && year % 4 == 0 || year % 400 == 0;
    37.     }
    38. }

    代码实现 - 总天数相减

    1. class Solution {
    2.     public int daysBetweenDates(String date1, String date2) {
    3.         return Math.abs(toDays(date1) - toDays(date2));
    4.     }
    6.     private int toDays(String date) {
    7.         int count = 0;
    8.         String[] s = date.split("-");
    9.         int y = Integer.parseInt(s[0]), m = Integer.parseInt(s[1]), d = Integer.parseInt(s[2]);
    10.         int[] days = {0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
    11.         days[2] += isLeapYear(y) ? 1 : 0;
    13.         for (int i = 1970; i < y; i++) {
    14.             count += isLeapYear(i) ? 366 : 365;
    15.         }
    16.         count += d;
    17.         for (int i = 1; i < m; i++) {
    18.             count += days[i];
    19.         }
    21.         return count;
    22.     }
    24.     private boolean isLeapYear(int year) {
    25.         return year % 4 == 0 && year % 100 != 0 || year % 400 == 0;
    26.     }
    27. }