Day of the Year - Practice Coding Problems

Day of the Year - Problem

Math String Easy

Day of the Year is a classic date calculation problem where you need to determine which day of the year a given date represents.

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

For example:
• January 1st is day 1
• February 1st is day 32 (31 days in January + 1)
• March 1st is day 60 in a leap year, or 59 in a regular year

The key challenge is handling leap years correctly, where February has 29 days instead of 28.

Input & Output

example_1.py — Basic Date

$ Input: "2019-01-09"

› Output: 9

💡 Note: January 9th is simply the 9th day of the year since January is the first month.

example_2.py — Cross Month Boundary

$ Input: "2019-02-10"

› Output: 41

💡 Note: February 10th = 31 days in January + 10 days in February = 41st day of the year.

example_3.py — Leap Year

$ Input: "2016-03-01"

› Output: 61

💡 Note: March 1st in leap year 2016 = 31 (Jan) + 29 (Feb in leap year) + 1 (Mar 1st) = 61st day.

Constraints

date.length == 10
date[4] == date[7] == '-'
All other characters in date are digits
date represents a calendar date between Jan 1st, 1900 and Dec 31st, 2019
The year is between 1900 and 2019

Visualization

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Understanding the Visualization

Create Month Lookup

Set up an array with days in each month: [31,28,31,30,31,30,31,31,30,31,30,31]

Parse the Date

Extract year, month, and day from the input string

Sum Complete Months

Add up all the days in months before the target month

Add Current Day

Add the day number within the current month

Handle Leap Years

If it's a leap year and we're past February, add one extra day

Key Takeaway

🎯 Key Insight: We only need to sum the days in completed months plus the current day, making this an O(1) operation since there are always exactly 12 months to consider at most.

Asked in

G Google 15 ⊞ Microsoft 12 a Amazon 8 Apple 6

The optimal solution uses a pre-computed array of days per month and directly calculates the result in O(1) time. The key insight is to sum the days in all completed months and add the current day number, while carefully handling leap years by adding an extra day when the month is after February.

Common Approaches

Approach	Time	Space	Notes
✓ Optimized Direct Calculation	O(1)	O(1)	Use pre-computed days per month array and direct calculation
Brute Force (Day by Day Counting)	O(m)	O(1)	Count every single day from January 1st to the target date

Optimized Direct Calculation — Algorithm Steps

Create an array with days per month: [31,28,31,30,31,30,31,31,30,31,30,31]
Parse the input date to get year, month, and day
Check if it's a leap year and adjust February if needed
Sum up all days in months before the target month
Add the current day number
Return the total

Visualization

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Step-by-Step Walkthrough

Setup Array

Create days array: [31,28,31,30,31,30,31,31,30,31,30,31]

Sum Months

Sum completed months: January (31 days)

Add Current Day

Add current day (10) to get 41

Code -

solution.c — C

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int dayOfYear(char* date) {
    int year, month, day;
    sscanf(date, "%d-%d-%d", &year, &month, &day);
    
    int days[] = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
    
    int result = day;
    
    // Add days from completed months
    for (int i = 0; i < month - 1; i++) {
        result += days[i];
    }
    
    // Add extra day for leap year if we've passed February
    if (month > 2 && year % 4 == 0 && (year % 100 != 0 || year % 400 == 0)) {
        result += 1;
    }
    
    return result;
}

int main() {
    char date[12];
    fgets(date, sizeof(date), stdin);
    date[strcspn(date, "\n")] = 0;
    if (date[0] == '"') {
        memmove(date, date + 1, strlen(date));
        date[strlen(date) - 1] = '\0';
    }
    printf("%d\n", dayOfYear(date));
    return 0;
}

Time & Space Complexity

Time Complexity

⏱️

O(1)

We only iterate through at most 12 months, which is constant time

✓ Linear Growth

Space Complexity

O(1)

Only using a fixed-size array and a few variables

✓ Linear Space

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Input & Output

Constraints

Visualization

Related Problems

Common Approaches

Optimized Direct Calculation — Algorithm Steps

Visualization

Code -

Time & Space Complexity

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