Problem #23

Problem link
Solution:

package main

import (
    "math"
    "container/list"
)

var writable []bool
var limit int
var abunList *list.List

func main() {
    abunList = list.New()
    limit = 28124
    writable = make([]bool, limit)
    for i := 1; i < limit; i++ {
        writable[i] = false
        if i < sumOfProperDivisors(i) {
            abunList.PushBack(i)
        }
    }

    checkWritables()

    sum := 0
    for i := 1; i < limit; i++ {
        if !writable[i] {
            sum += i
        }
    }
    println(sum)
}

func checkWritables() {
    for i := abunList.Front(); i != nil; i = i.Next() {
        for j := i; j != nil; j = j.Next() {
            if sum := i.Value.(int) + j.Value.(int); sum < limit {
                writable[sum] = true
            }
        }
    }
}

func sumOfProperDivisors(input int) int {
    //checking until the square root is enough
    //since a divisor less than the square root corresponds
    //the other divisor greater than the square root
    limit, sum := int(math.Sqrt(float64(input))), 0
    for i := 1; i <= limit; i++ {
        if input%i == 0 {
            if i == input/i {
                sum += i
            } else {
                sum += i + (input / i)
            }
        }
    }
    return sum - input
}

Result: 4179871
Time: 0m1.406s