Problem #44

Problem link
Solution:

package main

var arr []int

func main() {
    //2500 is learned after experiment. The array includes the result
    arr = make([]int, 2500)
    for i := range arr {
        arr[i] = (i * (3*i - 1)) / 2
    }
    min := int(^uint(0) >> 1)
    for i := 1; i < len(arr); i++ {
        for j := i + 1; j < len(arr); j++ {
            if isPentagonal(arr[j]-arr[i]) && isPentagonal(arr[j]+arr[i]) && arr[j]-arr[i] < min {
                min = arr[j] - arr[i]
            }
        }
    }
    println(min)
}

func isPentagonal(in int) bool {
    return search(in, 0, len(arr)-1)
}

func search(value, 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: 5482660
Time: 0m0.961s