Problem #19

Problem link
Solution:

package main

var months []int
var count int

func main() {
    months = make([]int, 12)

    //calculate first days of the months in 1900
    daysInMonths := []int{31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30}
    months[0] = 0
    for i := range daysInMonths {
        months[i+1] = (months[i] + daysInMonths[i]) % 7
    }

    count = 0
    for year := 1900; year < 2000; year++ {
        for i := range months {
            if (year%4 == 0 && i <= 1) || ((year+1)%4 == 0 && i > 1) {
                months[i] = (months[i] + 366) % 7
            } else {
                months[i] = (months[i] + 365) % 7
            }
            if months[i] == 6 {
                count++
            }
        }
    }
    println(count)
}

Result: 171
Time: 0m0.003s