https://leetcode.com/problems/day-of-the-week/

day-of-the-week.java

class Solution {
    public String dayOfTheWeek(int day, int month, int year) {
        
        int dayDiff = getDayDiff(day, month, year);
        int currDayDiff = getDayDiff(9, 2, 2020);
        int diff = dayDiff - currDayDiff;
        int shift = diff % 7;
        shift = (shift + 7) % 7;
        String[] dict = {"Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"};
        int currDayIndex = 0;
        return dict[ currDayIndex + shift];
    }
    
    private int getDayDiff(int day, int month, int year) {
        int res = 0;
        for (int y = 1971; y < year; y++) {
            res += (checkLeapYear(y) ? 366 : 365);
        }
        boolean isLeap = checkLeapYear(year);
        for (int m = 1; m < month; m++) {
            res += getDayCount(isLeap, m);
        }
        res += day;
        return res;
    }
    
    
    private int getDayCount(boolean isLeap, int month) {
        if (isLeap && month == 2) {
            return 29;
        } else if (month == 2) {
            return 28;
        }
        return ((month - 1) % 7) % 2 == 0 ? 31 : 30;
    }
    
    private boolean checkLeapYear(int year) {
        return (year % 400 == 0) || (year % 4 == 0 && year % 100 != 0);
    }
}