#1154

Easy

LeetCode Problem Workspace

Day of the Year

Calculate the day number of the year based on a given Gregorian calendar date in the format YYYY-MM-DD.

Problem Statement

You are given a string representing a date in the Gregorian calendar, formatted as YYYY-MM-DD. Your task is to return the day number of that year, where January 1st is the first day and December 31st is the last day of the year.

For example, given the date "2019-01-09", you should return 9 as it's the 9th day of the year. Pay special attention to leap years, as February may have 29 days instead of 28.

Examples

Example 1

Input: date = "2019-01-09"

Output: 9

Given date is the 9th day of the year in 2019.

Example 2

Input: date = "2019-02-10"

Output: 41

Example details omitted.

Constraints

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

Solution Approach

Date Parsing and Day Calculation

First, split the given string into its year, month, and day components. Convert the string values to integers for calculation. Then, calculate the total days by summing the days of all previous months in the year up to the given month and adding the day value from the current month.

Leap Year Adjustment

In leap years, February has one extra day. Before calculating the day of the year, check if the year is a leap year using the standard leap year rule: a year is a leap year if it is divisible by 4, except when divisible by 100, unless also divisible by 400.

Efficient Day Count with Month Length Array

Predefine an array representing the number of days in each month (accounting for leap years in February). Use this array to accumulate the days for all months up to the given month, and add the day of the current month to get the total.

Complexity Analysis

Metric	Value
Time	Depends on the final approach
Space	Depends on the final approach

The time complexity of this approach is O(1), as the operations are constant, involving simple array access and calculations. The space complexity is O(1) as only a fixed amount of data (month-day array) is used.

What Interviewers Usually Probe

Look for candidates who correctly handle leap years using conditional checks.
Evaluate understanding of how arrays can optimize counting days across months.
Ensure that candidates can break down the problem into small, manageable steps.

Common Pitfalls or Variants

Common pitfalls

Forgetting to account for leap years, which can cause an off-by-one error in February.
Incorrectly indexing the month, as arrays in programming are often zero-indexed, but months are one-indexed in this problem.
Not handling edge cases, such as dates in December or the very start of the year.

Follow-up variants

Handling different date formats, such as MM-DD-YYYY.
Calculating the day number for a different range of years, not limited to 1900–2019.
Optimizing the approach for larger datasets or multiple date calculations in one go.

FAQ

What is the day of the year problem?

The problem asks to calculate the day number of the year based on a given Gregorian date formatted as YYYY-MM-DD.

How do you handle leap years in the day of the year problem?

Leap years are handled by checking if the year is divisible by 4, not divisible by 100 unless also divisible by 400. If true, February has 29 days.

What should be considered when parsing a date in the day of the year problem?

Consider extracting the year, month, and day, and correctly managing the leap year condition for February to avoid errors in calculations.

How does the month-day array help in solving the day of the year problem?

The month-day array allows for an efficient lookup of the number of days in each month, including adjustments for leap years, making the calculation faster and easier.

What are common pitfalls in solving the day of the year problem?

Common pitfalls include failing to handle leap years, incorrect array indexing for months, and missing edge cases like the first and last days of the year.

terminal

Solution

Solution 1: Direct Calculation

According to the problem, the given date is in the Gregorian calendar, so we can directly calculate which day of the year it is.

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class Solution:
    def dayOfYear(self, date: str) -> int:
        y, m, d = (int(s) for s in date.split('-'))
        v = 29 if y % 400 == 0 or (y % 4 == 0 and y % 100) else 28
        days = [31, v, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31]
        return sum(days[: m - 1]) + d

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