Number of Days in a Month - Problem

Math Easy

Given a year and a month, return the number of days in that month.

You need to handle leap years correctly. A leap year occurs:

Every 4 years, BUT
NOT every 100 years, BUT
Every 400 years

For example, 2000 and 2004 are leap years, but 1900 is not.

Remember that February has 28 days in regular years and 29 days in leap years.

Input & Output

Example 1 — Leap Year February

$ Input: year = 2020, month = 2

› Output: 29

💡 Note: 2020 is a leap year (divisible by 4, not by 100), so February has 29 days

Example 2 — Regular Month

$ Input: year = 2019, month = 8

› Output: 31

💡 Note: August always has 31 days regardless of leap year

Example 3 — Non-Leap February

$ Input: year = 1900, month = 2

› Output: 28

💡 Note: 1900 is not a leap year (divisible by 100 but not by 400), so February has 28 days

Constraints

1583 ≤ year ≤ 2100
1 ≤ month ≤ 12

Visualization

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Asked in

G Google 15 M Microsoft 12 a Amazon 8

The key insight is that only February varies based on leap years - all other months have fixed day counts. The optimal approach uses either an array lookup with leap year checking or mathematical patterns. Time: O(1), Space: O(1)

Common Approaches

✓ Array Lookup

⏱️ Time: O(1) Space: O(1)

Store the number of days for each month in an array and use a helper function to determine if a year is a leap year. This approach is cleaner and more maintainable.

If-Else Chain

⏱️ Time: O(1) Space: O(1)

Use a series of if-else statements to handle each month case by case. For February, calculate if it's a leap year using the standard leap year rules.

Mathematical Formula

⏱️ Time: O(1) Space: O(1)

Apply mathematical rules directly: months 1,3,5,7,8,10,12 have 31 days, months 4,6,9,11 have 30 days, and February has 28/29 based on leap year calculation.

Array Lookup — Algorithm Steps

Step 1: Create array with days for each month (February as 28)
Step 2: Check if current year is leap year using helper function
Step 3: Adjust February to 29 days if leap year, return result

Visualization

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Step-by-Step Walkthrough

Array Lookup

Get days from predefined array

Leap Year Check

Check if February needs adjustment

Return Result

Return adjusted or original value

Code -

solution.c — C

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

int isLeapYear(int year) {
    return (year % 400 == 0) || (year % 4 == 0 && year % 100 != 0);
}

int solution(int year, int month) {
    int days[] = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
    
    if (month == 2 && isLeapYear(year)) {
        return 29;
    }
    return days[month - 1];
}

int main() {
    int year, month;
    scanf("%d", &year);
    scanf("%d", &month);
    int result = solution(year, month);
    printf("%d\n", result);
    return 0;
}

Time & Space Complexity

Time Complexity

⏱️

O(1)

Array lookup and leap year check are constant time operations

✓ Linear Growth

Space Complexity

O(1)

Fixed-size array of 12 elements regardless of input

✓ Linear Space

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Number of Days in a Month - Problem

Input & Output

Constraints

Visualization

Related Problems

Common Approaches

Array Lookup — Algorithm Steps

Visualization

Code -

Time & Space Complexity

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