Marc Lester Tan

This is rather easier if you can have access to a calendar to check if a certain year is a leap year, otherwise, you need to memorize this criteria to check if a year is a leap year:

year % 4 == 0 && year % 100 > 0 || year % 400 == 0

class Solution {
    fun numberOfDays(Y: Int, M: Int): Int {
        return when {
            M == 1 || M == 3 || M == 5 ||
                M == 7 || M == 8 || M == 10 || M == 12 -> 31
            else -> {
                if(M == 2) {
                    if(isLeapYear(Y)) 29 else 28
                }else 30
            }
        }
    }
    
    private fun isLeapYear(year: Int): Boolean = year % 4 == 0 && year % 100 > 0 || year % 400 == 0
}

Runtime: 112 ms, faster than 100.00% of Kotlin online submissions for Number of Days in a Month.
Memory Usage: 30.7 MB, less than 100.00% of Kotlin online submissions for Number of Days in a Month.