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