Problem #12

Problem link
Solution:

package main

import (
    "math"
)

func main() {
    divisors := 1
    triangleN := uint64(1)
    for i := 2; divisors < 500; i++ {
        triangleN += uint64(i)
        divisors = noOfDivisors(triangleN)
    }
    println(triangleN)
}

func noOfDivisors(number uint64) int {
    counter := 0
    limit := uint64(math.Sqrt(float64(number)))
    for i := 1; uint64(i) <= limit; i++ {
        if number%uint64(i) == 0 {
            counter += 2
        }
    }
    return counter
}

Result: 76576500
Time: 0m1.010s

Problem #42

Problem link
Solution:

package main

import (
    "io/ioutil"
    "strings"
)

var triangles []int

func main() {
    //read names
    fileBuf, err := ioutil.ReadFile("042_input.txt")
    if err != nil {
        panic(err.String())
    }
    fileStr := string(fileBuf)
    arr := strings.Split(fileStr, ",", -1)
    for i := range arr {
        arr[i] = strings.Split(arr[i], "\"", -1)[1]
    }

    //create an array of triangle numbers
    triangles = make([]int, 20)
    for i := range triangles {
        triangles[i] = (i * (i + 1)) / 2
    }

    //iterate over names to check
    result := 0
    for i := range arr {
        if isTriangle(alphabeticValue(arr[i])) {
            result++
        }
    }
    println(result)
}

func isTriangle(in int) bool {
    for i := range triangles {
        if triangles[i] == in {
            return true
        }
    }
    return false
}

func alphabeticValue(in string) int {
    sum := 0
    for i := range in {
        sum += int(in[i]) - int('A') + 1
    }
    return sum
}

Result: 162
Time: 0m0.009s

Problem #45

Problem link
Solution:

package main

var arr []int64 //pentagonals

func main() {
    arr = make([]int64, 80000)
    for i := 1; i < len(arr); i++ {
        arr[i] = int64(i) * (3*int64(i) - 1) / 2
    }
    var tmp int64
    i := 144
loop:
    for {
        tmp = int64(i) * int64(2*i-1)
        if isPentagonal(tmp) { //every hexagonal is also triangle
            println(tmp)
            break loop
        }
        i++
    }
}

func isPentagonal(in int64) bool {
    return search(in, 1, len(arr)-1)
}

func search(value int64, low, high int) bool {
    if high < low {
        return false
    }
    mid := low + (high-low)/2
    if arr[mid] > value {
        return search(value, low, mid-1)
    } else if arr[mid] < value {
        return search(value, mid+1, high)
    }
    return true
}

Result: 1533776805
Time: 0m0.035s