day-of-the-week.cpp

// Time:  O(1)
// Space: O(1)

class Solution {
public:
    string dayOfTheWeek(int day, int month, int year) {
        static const vector<string> DAYS = {"Sunday", "Monday", "Tuesday", "Wednesday",
                                            "Thursday", "Friday", "Saturday"};

        // Zeller Formula
        if (month < 3) {
            month += 12;
            --year;
        }
        int c = year / 100, y = year % 100;
        const auto& w = ((c / 4 - 2 * c + y + y / 4 + 13 * (month + 1) / 5 + day - 1) % 7 + 7) % 7;
        return DAYS[w];
    }
};