What is the time complexity of Day of the Year?

The optimal solution has a time complexity of O(1).

What data structures are used to solve Day of the Year?

This problem uses Math, String. Understanding these concepts is essential for solving this Easy problem efficiently.

What are the key insights for solving Day of the Year?

**Precise Leap Year Calculation**: The cornerstone of this problem is correctly implementing the Gregorian calendar's leap year rules: `(year % 4 == 0 and year % 100 != 0) or (year % 400 == 0)`. Failing to capture this nuance, especially the centurial exception, will lead to incorrect results for February's length in specific years. **Month Length Pre-computation/Lookup**: Storing the number of days for each month in an array (`days_in_month`) provides a clean and efficient lookup mechanism. Instead of using `if/else if` cascades for each month, a simple array access `days_in_month[i]` keeps the code concise and readable, especially after dynamically adjusting February's value for leap years.

Which companies ask LeetCode 1154. Day of the Year in interviews?

This problem is frequently asked at ZScaler.

JavaScript is required for the full interactive experience. However, all content is accessible without JavaScript.

Day of the Year - LeetCode 1154 Solution

Day of the Year - Complete Solution Guide

Day of the Year is LeetCode problem 1154, a Easy level challenge. This complete guide provides step-by-step explanations, multiple solution approaches, and optimized code in python3, java, cpp, c.

Problem Statement

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.

Detailed Explanation

This problem challenges us to calculate the sequential day number within a given year, based on a standard Gregorian calendar date. The input `date` comes as a string in `YYYY-MM-DD` format, like "2019-01-09". Our task is to return an integer representing which day of the year that date falls on. For example, "2019-01-09" should yield 9, as it's the 9th day. "2019-02-10" would require summing January's 31 days and then adding 10 for February, resulting in 41.

Solution Approach

The provided solution adopts a highly pragmatic, direct approach: simulate the counting of days. First, it efficiently parses the input `date` string by splitting it at the hyphens and converting the resulting year, month, and day components into integers. This sets up our core variables. Next, it initializes an array, `days_in_month`, storing the standard number of days for each month in a non-leap year (e.g., February starts with 28 days). This array acts as a lookup table. The critical logic follows: a conditional check determines if the extracted `year` is a leap year using the standard rules: divisible by 4 but not by 100, OR divisible by 400. If it's a leap year, the solution immediately updates the `days_in_month` array, setting February's entry (index 1) to 29. With the correct month lengths established, the algorithm then iterates from January (index 0) up to the *month prior* to our target month. In each iteration, it accumulates the total days from these full preceding months. Finally, it adds the `day` component from the input date itself. This sum represents the total day number of the year. This method works beautifully because it mimics exactly how one would manually count days, making it intuitively understandable and robust for the given constraints.

Step-by-Step Algorithm

Step 1: Parse the input date string into year, month, and day integers.
Step 2: Initialize an array `daysInMonth` to store the number of days in each month (starting from January).
Step 3: Check if the year is a leap year using the leap year rule (divisible by 4, but not by 100 unless also divisible by 400). If it's a leap year, set the number of days in February to 29.
Step 4: Iterate through the `daysInMonth` array from January to the month before the input month, summing the number of days in each month. This gives the total number of days passed before the input month.
Step 5: Add the input day to the accumulated sum to get the final day of the year.
Step 6: Return the final day of the year.

Key Insights

**Precise Leap Year Calculation**: The cornerstone of this problem is correctly implementing the Gregorian calendar's leap year rules: `(year % 4 == 0 and year % 100 != 0) or (year % 400 == 0)`. Failing to capture this nuance, especially the centurial exception, will lead to incorrect results for February's length in specific years.
**Month Length Pre-computation/Lookup**: Storing the number of days for each month in an array (`days_in_month`) provides a clean and efficient lookup mechanism. Instead of using `if/else if` cascades for each month, a simple array access `days_in_month[i]` keeps the code concise and readable, especially after dynamically adjusting February's value for leap years.
**Iterative Summation for Preceding Months**: The solution's strategy of summing days from full months *before* the target month, then adding the target day, is a straightforward and explicit way to count. It avoids complex date arithmetic libraries and directly translates the problem's request into a simple loop, ensuring correctness by adding exact integer values.

Complexity Analysis

Time Complexity: O(1)

Space Complexity: O(1)

Topics

This problem involves: Math, String.

Companies

Asked at: ZScaler.

Day of the Year | LeetCode 1154

Easy

Math String

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

Solution Code

Available in:

class Solution {
    public int dayOfYear(String date) {
        int year = Integer.parseInt(date.substring(0, 4));
        int month = Integer.parseInt(date.substring(5, 7));
        int day = Integer.parseInt(date.substring(8, 10));

        int[] daysInMonth = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};

        if ((year % 4 == 0 && year % 100 != 0) || (year % 400 == 0)) {
            daysInMonth[1] = 29;
        }

        int dayOfYear = 0;
        for (int i = 0; i < month - 1; i++) {
            dayOfYear += daysInMonth[i];
        }
        dayOfYear += day;

        return dayOfYear;
    }
}

View Full Solution on GitHubGitHub Solution View on LeetCodeLeetCode

Problem Info

Description

Problem 1154: Day of the Year - Detailed explanation and solution approaches.

Topics

Math String

Companies

ZScaler

Stats

Difficulty:Easy

Languages:4

Companies:1

Problem #:1154

Problem Explanation

💡

Think Before You Code

🤔 Ask Yourself

→What is the input? What output do I need?
→Can I solve a smaller version of this problem first?
→What patterns have I seen in similar problems?

✏️ Try This First

Before looking at the solution, try to solve it on paper with a simple example!

🚀 Try on LeetCode

Solution Approach

📋

Step-by-Step Solution

6 steps

1
Step 1: Parse the input date string into year, month, and day integers.
2
Step 2: Initialize an array `daysInMonth` to store the number of days in each month (starting from January).
3
Step 3: Check if the year is a leap year using the leap year rule (divisible by 4, but not by 100 unless also divisible by 400). If it's a leap year, set the number of days in February to 29.
4
Step 4: Iterate through the `daysInMonth` array from January to the month before the input month, summing the number of days in each month. This gives the total number of days passed before the input month.
5
Step 5: Add the input day to the accumulated sum to get the final day of the year.
6
Step 6: Return the final day of the year.

💡

Pro Tip for Students

Try tracing through each step with a small example on paper before coding. Understanding "why" each step happens is more important than memorizing the code!

⚡

Complexity Analysis

⏱️

Time Complexity

O(1)

⚡ Constant time - Super fast! Same speed regardless of input size.

💾

Space Complexity

O(1)

💪 Constant space - Uses same memory regardless of input!

📝

Why?

The algorithm performs a fixed number of operations regardless of the input date. The loop iterates at most 12 times (for months). The array `daysInMonth` has a constant size. Therefore, both time and space complexity are constant.

📖New to Big O? Click to learn more

Big O Notation - Quick Guide

O(1)Best - Instant

O(log n)Excellent - Binary Search

O(n)Good - One loop

O(n log n)OK - Sorting

O(n²)Slow - Nested loops

O(2ⁿ)Very Slow - Recursion

Alternative Approaches

Using Date Libraries

Many programming languages have built-in date and time libraries. These libraries often provide functions to directly calculate the day of the year.

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

Trade-offs: Using a date library can be more concise and potentially more efficient, but it might require familiarity with the specific library's API. The manual approach is more educational and demonstrates the underlying logic.

✨

Key Insights

1**Precise Leap Year Calculation**: The cornerstone of this problem is correctly implementing the Gregorian calendar's leap year rules: `(year % 4 == 0 and year % 100 != 0) or (year % 400 == 0)`. Failing to capture this nuance, especially the centurial exception, will lead to incorrect results for February's length in specific years.
2**Month Length Pre-computation/Lookup**: Storing the number of days for each month in an array (`days_in_month`) provides a clean and efficient lookup mechanism. Instead of using `if/else if` cascades for each month, a simple array access `days_in_month[i]` keeps the code concise and readable, especially after dynamically adjusting February's value for leap years.
3**Iterative Summation for Preceding Months**: The solution's strategy of summing days from full months *before* the target month, then adding the target day, is a straightforward and explicit way to count. It avoids complex date arithmetic libraries and directly translates the problem's request into a simple loop, ensuring correctness by adding exact integer values.

📚

Key Terms Explained

For Students

String

A sequence of characters. Like a word or sentence in programming!

Edge Cases

Edge case 1: January 1st. This is handled correctly as the loop doesn't run and only the day is added, resulting in day 1.
Edge case 2: Leap years. The code correctly identifies leap years and adjusts the number of days in February, ensuring the correct day of the year is calculated.
Edge case 3: December 31st. The loop iterates through all months, correctly summing all days, and adds the day (31), resulting in the correct day of the year.

⚠️

Common Mistakes to Avoid

✗Common mistake 1: Incorrect leap year calculation. Forgetting to handle the exception for years divisible by 100 but not 400 is a frequent mistake.

✗Common mistake 2: Off-by-one errors in the loop. Incorrectly handling the starting and ending indices of the loop can lead to incorrect day of year calculations.

✅

Learning Tip

Every mistake is a learning opportunity! When you make an error, understand why it happened before moving on.

🌍

Where is This Used in Real Life?

💼Real-world scenario 1: Calculating the number of days between two dates, useful for financial applications (e.g., calculating interest accrual), or scheduling and logistics.

🏦Real-world scenario 2: Data analysis and reporting involving date information, such as determining the distribution of events throughout the year.

💡Understanding real-world applications helps you see the "why" behind algorithms. It's not just about passing interviews—these concepts power the apps and websites you use every day!

Author's Notes

💡 Key Insight

When dealing with arrays, always consider if sorting would simplify the problem. Many array problems have O(n) solutions using hash maps or two-pointer technique.

🎯 Interview Tip

Don't rush. Explain your thought process clearly. Easy problems are often about communication.

⚠️ Watch Out

Always test your solution with edge cases: empty input, single element, and boundary values.

Written by Saswata Mondal

Software Engineer • Algorithms Enthusiast • Interview Coach

👨‍💻

About the Author

👨‍💻

Saswata MondalAuthor400+ LC Problems

B.Tech CSE, IEM Kolkata | LeetCode Rating: 1672 (Top 15%)

Certified: IBM Java Developer, NPTEL Programming in Java

Last updated: August 14, 2024LeetCode Profile GitHub

This solution has been crafted based on my experience solving 400+ LeetCode problems. Each explanation is designed to help you understand the underlying concepts, not just memorize the code.

Community Tips & Insights

In a real interview, always clarify constraints first. Ask about input size and value ranges.

@TechLeadJul 13

Don't forget to handle the edge case when the input is empty. Many candidates miss this in interviews.

@DevExpert92Jul 6

❓

Frequently Asked Questions

How do I handle string manipulation efficiently?

In languages with immutable strings (Java, Python, C#), use StringBuilder/list to avoid O(n) concatenation cost. Consider using character arrays for in-place modifications. For pattern matching, consider KMP or Rabin-Karp for O(n+m) complexity.

How should I approach easy-level algorithm problems?

Easy problems build foundational skills. Focus on: 1) Understanding the problem completely before coding, 2) Writing clean, readable code, 3) Handling all edge cases (empty input, single element, etc.), 4) Analyzing time and space complexity. Don't skip complexity analysis even for simple solutions.

How should I practice algorithm problems effectively?

Focus on understanding patterns rather than memorizing solutions. After solving a problem, review optimal solutions and understand the intuition. Group similar problems to recognize patterns. Practice explaining your approach out loud. Review problems after a few days to test retention.

What is the optimal approach for "Day of the Year"?

For this easy-level Math problem, the key is to identify the core pattern. Analyze the input constraints to determine acceptable time complexity. Consider whether string techniques can optimize the solution. Always handle edge cases and validate your approach with examples before coding.

🤖 Need help with this problem?