func isPerfectSquare(num int) bool {
	beg, end := 1, num+1
	for beg < end {
		mid := beg + (end-beg)/2
		div := num / mid
		if div == mid && div*mid == num { // 5 / 2 == 2, but 2 * 2 != 5
			return true
		} else if div < mid {
			end = mid
		} else if mid <= div {
			beg = mid + 1
		}
	}
	return false
}