Solution 1: Mathematics

First, we define a function isLeapYear(year) to determine whether the given year year is a leap year. If it is a leap year, return true, otherwise return false.

Next, we define another function daysInMonth(year, month) to calculate the total number of days in the given year year and month month. We can use an array days to store the number of days in each month, where days[1] represents the number of days in February. If it is a leap year, it is $29$ days, otherwise it is $28$ days.

Then, we define another function calcDays(date) to calculate the number of days from the given date date to 1971-01-01. We can use date.split("-") to split the date date into year year, month month, and day day by -. Then we can use a loop to calculate the total number of days from 1971 to year, then calculate the total number of days from January to month, and finally add day days.

Finally, we only need to return the absolute value of calcDays(date1) - calcDays(date2).

The time complexity is $O (y + m)$ , where $y$ represents the number of years from the given date to 1971-01-01, and $m$ represents the number of months of the given date. The space complexity is $O (1)$ .

Python3

class Solution:
    def daysBetweenDates(self, date1: str, date2: str) -> int:
        def isLeapYear(year: int) -> bool:
            return year % 4 == 0 and (year % 100 != 0 or year % 400 == 0)

        def daysInMonth(year: int, month: int) -> int:
            days = [
                31,
                28 + int(isLeapYear(year)),
                31,
                30,
                31,
                30,
                31,
                31,
                30,
                31,
                30,
                31,
            ]
            return days[month - 1]

        def calcDays(date: str) -> int:
            year, month, day = map(int, date.split("-"))
            days = 0
            for y in range(1971, year):
                days += 365 + int(isLeapYear(y))
            for m in range(1, month):
                days += daysInMonth(year, m)
            days += day
            return days

        return abs(calcDays(date1) - calcDays(date2))

Java

class Solution {
    public int daysBetweenDates(String date1, String date2) {
        return Math.abs(calcDays(date1) - calcDays(date2));
    }

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

    private int daysInMonth(int year, int month) {
        int[] days = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
        days[1] += isLeapYear(year) ? 1 : 0;
        return days[month - 1];
    }

    private int calcDays(String date) {
        int year = Integer.parseInt(date.substring(0, 4));
        int month = Integer.parseInt(date.substring(5, 7));
        int day = Integer.parseInt(date.substring(8));
        int days = 0;
        for (int y = 1971; y < year; ++y) {
            days += isLeapYear(y) ? 366 : 365;
        }
        for (int m = 1; m < month; ++m) {
            days += daysInMonth(year, m);
        }
        days += day;
        return days;
    }
}

C++

class Solution {
public:
    int daysBetweenDates(string date1, string date2) {
        return abs(calcDays(date1) - calcDays(date2));
    }

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

    int daysInMonth(int year, int month) {
        int days[12] = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
        days[1] += isLeapYear(year);
        return days[month - 1];
    }

    int calcDays(string date) {
        int year = stoi(date.substr(0, 4));
        int month = stoi(date.substr(5, 2));
        int day = stoi(date.substr(8, 2));
        int days = 0;
        for (int y = 1971; y < year; ++y) {
            days += 365 + isLeapYear(y);
        }
        for (int m = 1; m < month; ++m) {
            days += daysInMonth(year, m);
        }
        days += day;
        return days;
    }
};

Go

func daysBetweenDates(date1 string, date2 string) int {
    return abs(calcDays(date1) - calcDays(date2))
}

func isLeapYear(year int) bool {
    return year%4 == 0 && (year%100 != 0 || year%400 == 0)
}

func daysInMonth(year, month int) int {
    days := [12]int{31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31}
    if isLeapYear(year) {
        days[1] = 29
    }
    return days[month-1]
}

func calcDays(date string) int {
    year, _ := strconv.Atoi(date[:4])
    month, _ := strconv.Atoi(date[5:7])
    day, _ := strconv.Atoi(date[8:])
    days := 0
    for y := 1971; y < year; y++ {
        days += 365
        if isLeapYear(y) {
            days++
        }
    }
    for m := 1; m < month; m++ {
        days += daysInMonth(year, m)
    }
    days += day
    return days
}

func abs(x int) int {
    if x < 0 {
        return -x
    }
    return x
}

TypeScript

function daysBetweenDates(date1: string, date2: string): number {
    return Math.abs(calcDays(date1) - calcDays(date2));
}

function isLeapYear(year: number): boolean {
    return year % 4 === 0 && (year % 100 !== 0 || year % 400 === 0);
}

function daysOfMonth(year: number, month: number): number {
    const days = [31, isLeapYear(year) ? 29 : 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31];
    return days[month - 1];
}

function calcDays(date: string): number {
    let days = 0;
    const [year, month, day] = date.split('-').map(Number);
    for (let y = 1971; y < year; ++y) {
        days += isLeapYear(y) ? 366 : 365;
    }
    for (let m = 1; m < month; ++m) {
        days += daysOfMonth(year, m);
    }
    days += day - 1;
    return days;
}

Problem