DayOfTheWeek.java

package com.smlnskgmail.jaman.leetcodejava.easy;

// https://leetcode.com/problems/day-of-the-week/
public class DayOfTheWeek {

    private static final String[] DAY_OF_WEEK = new String[]{
            "Sunday",
            "Monday",
            "Tuesday",
            "Wednesday",
            "Thursday",
            "Friday",
            "Saturday"
    };
    private static final int[] MONTHS = new int[]{0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};

    private int day;
    private final int month;
    private final int year;

    public DayOfTheWeek(int day, int month, int year) {
        this.day = day;
        this.month = month;
        this.year = year;
    }

    public String solution() {
        for (int i = 1971; i < year; i++) {
            day += isLeap(i) ? 366 : 365;
        }
        if (month > 2 && isLeap(year)) {
            day++;
        }
        for (int i = 1; i < month; i++) {
            day += MONTHS[i];
        }
        return DAY_OF_WEEK[(day + 4) % 7];
    }

    public boolean isLeap(long year) {
        return (year % 4 == 0 && year % 100 != 0) || year % 400 == 0;
    }

}