LeetCode 1185: Day of the Week Solution

Master LeetCode problem 1185 (Day of the Week), a easy challenge, with our optimized solutions in Java, C++, and Python. Explore detailed explanations, test your code in our interactive editor, and prepare for coding interviews.

1185. Day of the Week

Easy

Math

Problem Explanation

Explanation

To solve this problem, we can use Zeller's Congruence algorithm to calculate the day of the week for a given date. The algorithm involves a formula that takes into account the day, month, and year of the date and returns a number corresponding to the day of the week. We can then map this number to the actual day name.

Algorithm

Adjust the month and year for Zeller's Congruence formula:
- If the month is January or February, treat them as months 13 and 14 of the previous year.
- Adjust the year accordingly.
Apply Zeller's Congruence formula:
- Calculate the day of the week using the formula: [ h = (q + \left\lfloor \frac{13(m+1)}{5} \right\rfloor + K + \left\lfloor \frac{K}{4} \right\rfloor + \left\lfloor \frac{J}{4} \right\rfloor - 2J) \mod 7 ] where:
  - ( h ) is the day of the week (0 for Saturday, 1 for Sunday, ..., 6 for Friday)
  - ( q ) is the day of the month
  - ( m ) is the adjusted month
  - ( K ) is the year of the century (year % 100)
  - ( J ) is the zero-based century (year / 100)
Map the calculated day of the week value to the corresponding day name.

Time Complexity

The time complexity of this algorithm is O(1) as it involves a fixed number of arithmetic operations.

Space Complexity

The space complexity is O(1) as we are using only a constant amount of extra space.

Solution Code

class Solution {
    public String dayOfTheWeek(int day, int month, int year) {
        String[] daysOfWeek = {"Saturday", "Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday"};
        
        if (month < 3) {
            month += 12;
            year--;
        }
        
        int q = day;
        int m = month;
        int K = year % 100;
        int J = year / 100;
        
        int h = (q + (13 * (m + 1) / 5) + K + (K / 4) + (J / 4) - 2 * J) % 7;
        
        return daysOfWeek[(h + 5) % 7]; // Adjust for Zeller's Congruence result
    }
}

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