Day of the Year - Practice Coding Problems

Day of the Year - Problem

Math String Easy

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

The day number represents which day of the year it is. For example, January 1st is day 1, January 2nd is day 2, and so on.

Note: You need to consider leap years when February has 29 days instead of 28.

Input & Output

Example 1 — Early February

$ Input: date = "2019-02-10"

› Output: 41

💡 Note: January has 31 days, so February 10th is the 31 + 10 = 41st day of the year

Example 2 — Leap Year March

$ Input: date = "2020-03-01"

› Output: 61

💡 Note: 2020 is a leap year. January (31) + February (29) + March 1st = 31 + 29 + 1 = 61

Example 3 — Year Start

$ Input: date = "2021-01-01"

› Output: 1

💡 Note: January 1st is always the 1st day of any year

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

Visualization

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Understanding the Visualization

Input

Date string in YYYY-MM-DD format

Process

Count days from January 1st to given date

Output

Day number (1-366) within the year

Key Takeaway

🎯 Key Insight: Use prefix sums to avoid recalculating cumulative days for each month

Asked in

M Microsoft 15 Apple 8

The key insight is to use cumulative day counts for each month and handle leap years properly. Best approach is prefix sum array with O(1) lookup. Time: O(1), Space: O(1)

Common Approaches

Approach	Time	Space	Notes
✓ Prefix Sum Array	O(1)	O(1)	Use precomputed cumulative days array for instant lookup
Day-by-Day Counting	O(m)	O(1)	Count each day from January 1st to the target date

Prefix Sum Array — Algorithm Steps

Create prefix sum array of days for each month
Handle leap year by adjusting February
Return prefix[month-1] + day

Visualization

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Step-by-Step Walkthrough

Prefix Array

Days accumulated before each month

Lookup

Get prefix[1] = 31 days before February

Add Day

31 + 10 = 41st day of year

Code -

solution.c — C

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

int solution(char* date) {
    int year, month, day;
    sscanf(date, "%d-%d-%d", &year, &month, &day);
    
    // Cumulative days before each month starts (non-leap year)
    int prefixDays[] = {0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334};
    
    // Check if it's a leap year and adjust for months after February
    int isLeap = (year % 4 == 0 && year % 100 != 0) || (year % 400 == 0);
    int leapAdjustment = (isLeap && month > 2) ? 1 : 0;
    
    return prefixDays[month - 1] + day + leapAdjustment;
}

int main() {
    char date[12];
    fgets(date, sizeof(date), stdin);
    date[strcspn(date, "\n")] = 0;
    printf("%d\n", solution(date));
    return 0;
}

Time & Space Complexity

Time Complexity

⏱️

O(1)

Direct array access and simple arithmetic operations

✓ Linear Growth

Space Complexity

O(1)

Fixed-size array of 12 elements regardless of input

✓ Linear Space

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Input & Output

Constraints

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Common Approaches

Prefix Sum Array — Algorithm Steps

Visualization

Code -

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