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Day of the Year

Easy

Asked by:

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Topics:

ArraysStrings

Given a string date representing a Gregorian calendar date formatted as YYYY-MM-DD, return the day number of the year.

Example 1:

Input: date = "2019-01-09"
Output: 9
Explanation: Given date is the 9th day of the year in 2019.

Example 2:

Input: date = "2019-02-10"
Output: 41

Constraints:

date.length == 10
date[4] == date[7] == '-', and all other date[i]'s are digits
date represents a calendar date between Jan 1^st, 1900 and Dec 31^st, 2019.

Try coding it in LeetCode

Day of the Year

Easy

Asked by:

7 views

Topics:

ArraysStrings

Given a string date representing a Gregorian calendar date formatted as YYYY-MM-DD, return the day number of the year.

Example 1:

Input: date = "2019-01-09"
Output: 9
Explanation: Given date is the 9th day of the year in 2019.

Example 2:

Input: date = "2019-02-10"
Output: 41

Constraints:

date.length == 10
date[4] == date[7] == '-', and all other date[i]'s are digits
date represents a calendar date between Jan 1^st, 1900 and Dec 31^st, 2019.

Try coding it in LeetCode

Solution

Clarifying Questions

When you get asked this question in a real-life environment, it will often be ambiguous (especially at FAANG). Make sure to ask these questions in that case:

What is the expected input format for the date? (e.g., YYYY-MM-DD, MM/DD/YYYY)?
Is the input date guaranteed to be a valid date within a reasonable range (e.g., no dates before the Gregorian calendar or far into the future)?
Should I handle leap years according to the Gregorian calendar rules?
Is it possible for the input to be null, empty, or invalid in some way, and if so, what should the function return in those cases?
Should I validate the input date for correctness before calculating the day of the year?

Brute Force Solution

Approach

The goal is to figure out which day of the year a given date is. The brute force method involves manually counting the days. We start from the beginning of the year and add days until we reach the given date.

Here's how the algorithm would work step-by-step:

Start by assuming the date is January 1st, which is day 1.
Check the month of the given date.
For each month before the target month, add the number of days in that month to the day count.
Remember to check if it's a leap year when you get to February and adjust the number of days accordingly.
Finally, add the day of the month from the given date to the running total.
The result is the day of the year for the given date.

Code Implementation

def day_of_the_year_brute_force(year_string, month_string, day_string):
    year = int(year_string)
    month = int(month_string)
    day = int(day_string)

    day_of_year = 0
    days_in_month = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31]

    # Check if it's a leap year
    if (year % 4 == 0 and year % 100 != 0) or (year % 400 == 0):

        days_in_month[1] = 29

    # Sum the days of previous months
    for month_index in range(month - 1):

        day_of_year += days_in_month[month_index]

    # Add the days of the current month
    day_of_year += day

    return day_of_year

Big(O) Analysis

Time Complexity

O(1) – The algorithm iterates through a maximum of 12 months (January to December). The number of operations is therefore bounded by a constant, regardless of the input date. The leap year check and addition of the day of the month are also constant time operations. Therefore, the time complexity is O(1).

Space Complexity

O(1) – The algorithm described uses a fixed number of variables, such as the day count and potentially temporary storage for the number of days in each month, for calculation. The number of months is constant (12), and the leap year check doesn't introduce additional data structures dependent on the input year. The space used is independent of the input date, thus the auxiliary space complexity is constant. Therefore, the space complexity is O(1).

Optimal Solution

Approach

To find the day of the year, we'll calculate how many days have passed in each month before the given date. A key optimization is to pre-calculate the number of days in each month and account for leap years efficiently.

Here's how the algorithm would work step-by-step:

Start with a list of how many days are in each month during a regular year.
Figure out if the year is a leap year. If it is, change the number of days in February to account for the extra day.
Add up all the days for each month that comes before the month in the input date.
Add the day of the month to that sum to get the final day of the year.

Code Implementation

def day_of_the_year(date): 
    year = int(date[:4])
    month = int(date[5:7])
    day = int(date[8:])

    days_in_month = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31]

    # Adjust for leap year.
    if (year % 4 == 0 and year % 100 != 0) or (year % 400 == 0):
        days_in_month[1] = 29

    total_days = 0

    # Sum days from previous months
    for i in range(month - 1):
        total_days += days_in_month[i]

    # Add the day of the current month.
    total_days += day

    return total_days

Big(O) Analysis

Time Complexity

O(1) – The solution involves calculating the day of the year by summing the days of the months preceding the given date. It initializes a fixed-size array representing the days in each month, which has a constant size. Determining if the year is a leap year is a constant time operation. Summing the days up to the target month is also bounded by the number of months, which is at most 12. Therefore, the number of operations doesn't scale with any input size and remains constant, making the time complexity O(1).

Space Complexity

O(1) – The solution uses a fixed-size array (days in each month) and a few integer variables to store the day, month, year, the leap year flag, and the running sum of days. The size of the array representing days in each month is constant (always 12 elements), and the number of integer variables remains constant regardless of the input date. Therefore, the auxiliary space used by the algorithm is constant, leading to a space complexity of O(1).

Edge Cases

Case	How to Handle
Date string is null or empty	Return an appropriate error code or exception indicating invalid input.
Date string is in an invalid format	Catch any parsing exceptions and return an error indicating the incorrect date format.
Invalid year (e.g., negative year or year 0)	Validate the year is a positive integer and return an error if not.
Invalid month (e.g., month 0 or month > 12)	Validate the month is within the range of 1 to 12 and return an error if not.
Invalid day (e.g., day 0 or day greater than the maximum days for the given month)	Validate the day is within the valid range for the given month and year, considering leap years, and return an error if not.
Leap year (February 29th)	Account for February having 29 days in leap years by correctly determining if the year is a leap year and adjusting the days accordingly.
Maximum valid date (e.g., '9999-12-31')	Ensure the algorithm handles the largest possible valid date without overflow or incorrect calculations.
Integer overflow in day calculation	Use appropriate data types (e.g., long) to prevent potential integer overflow during the summation of days.

Solution

Clarifying Questions

When you get asked this question in a real-life environment, it will often be ambiguous (especially at FAANG). Make sure to ask these questions in that case:

What is the expected input format for the date? (e.g., YYYY-MM-DD, MM/DD/YYYY)?
Is the input date guaranteed to be a valid date within a reasonable range (e.g., no dates before the Gregorian calendar or far into the future)?
Should I handle leap years according to the Gregorian calendar rules?
Is it possible for the input to be null, empty, or invalid in some way, and if so, what should the function return in those cases?
Should I validate the input date for correctness before calculating the day of the year?

Brute Force Solution

Approach

Here's how the algorithm would work step-by-step:

Start by assuming the date is January 1st, which is day 1.
Check the month of the given date.
For each month before the target month, add the number of days in that month to the day count.
Remember to check if it's a leap year when you get to February and adjust the number of days accordingly.
Finally, add the day of the month from the given date to the running total.
The result is the day of the year for the given date.

Code Implementation

def day_of_the_year_brute_force(year_string, month_string, day_string):
    year = int(year_string)
    month = int(month_string)
    day = int(day_string)

    day_of_year = 0
    days_in_month = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31]

    # Check if it's a leap year
    if (year % 4 == 0 and year % 100 != 0) or (year % 400 == 0):

        days_in_month[1] = 29

    # Sum the days of previous months
    for month_index in range(month - 1):

        day_of_year += days_in_month[month_index]

    # Add the days of the current month
    day_of_year += day

    return day_of_year

Big(O) Analysis

Time Complexity

Space Complexity

Optimal Solution

Approach

Here's how the algorithm would work step-by-step:

Start with a list of how many days are in each month during a regular year.
Figure out if the year is a leap year. If it is, change the number of days in February to account for the extra day.
Add up all the days for each month that comes before the month in the input date.
Add the day of the month to that sum to get the final day of the year.

Code Implementation

def day_of_the_year(date): 
    year = int(date[:4])
    month = int(date[5:7])
    day = int(date[8:])

    days_in_month = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31]

    # Adjust for leap year.
    if (year % 4 == 0 and year % 100 != 0) or (year % 400 == 0):
        days_in_month[1] = 29

    total_days = 0

    # Sum days from previous months
    for i in range(month - 1):
        total_days += days_in_month[i]

    # Add the day of the current month.
    total_days += day

    return total_days

Big(O) Analysis

Time Complexity

Space Complexity

Edge Cases

Case	How to Handle
Date string is null or empty	Return an appropriate error code or exception indicating invalid input.
Date string is in an invalid format	Catch any parsing exceptions and return an error indicating the incorrect date format.
Invalid year (e.g., negative year or year 0)	Validate the year is a positive integer and return an error if not.
Invalid month (e.g., month 0 or month > 12)	Validate the month is within the range of 1 to 12 and return an error if not.
Invalid day (e.g., day 0 or day greater than the maximum days for the given month)	Validate the day is within the valid range for the given month and year, considering leap years, and return an error if not.
Leap year (February 29th)	Account for February having 29 days in leap years by correctly determining if the year is a leap year and adjusting the days accordingly.
Maximum valid date (e.g., '9999-12-31')	Ensure the algorithm handles the largest possible valid date without overflow or incorrect calculations.
Integer overflow in day calculation	Use appropriate data types (e.g., long) to prevent potential integer overflow during the summation of days.