pad.go
func zfillString(src string, z int) string {
	size := len(src)
	if size >= z {
		return src
	}

	fill := strings.Repeat("0", z-size)
	if strings.HasPrefix(src, "-") {
		return fmt.Sprintf("-%s%s", fill, src[1:])
	}
	return fmt.Sprintf("%s%s", fill, src)
}
func zfillString(src string, z int) string {
	size := len(src)
	if size >= z {
		return src
	}

	fill := strings.Repeat("0", z-size)
	if strings.HasPrefix(src, "-") {
		return fmt.Sprintf("-%s%s", fill, src[1:])
	}
	return fmt.Sprintf("%s%s", fill, src)
}
// Left pads a string to a given with, using "0".
// If the string begins with a negative "-" character, then
// padding is inserted between the "-" and the remaining characters.