1185. Day of the Week 

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class Solution {
    private static final int[] DAYS_PER_MONTH = {0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
    private static final String[] DAY_OF_THE_WEEK = {"Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"};

    public String dayOfTheWeek(int day, int month, int year) {
        int resultDay = 4; // start from 1970-12-31, this day is Thursday

        for (int i = 1971; i < year; i++) {
            resultDay += isLeapYear(i) ? 366 : 365; // 注意此处是 i
        }
        for (int i = 1; i < month; i++) {
            resultDay += DAYS_PER_MONTH[i];
            if (i == 2 && isLeapYear(year)) {
                resultDay += 1;
            }
        }
        resultDay += day;

        return DAY_OF_THE_WEEK[resultDay % 7];
    }

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

References

1185. Day of the Week