题目： 判断有效的 PIN 字符串

描述： ATM machines allow 4 or 6 digit PIN codes and PIN codes cannot contain anything but exactly 4 digits or exactly 6 digits.

If the function is passed a valid PIN string, return true, else return false.

Examples (Input --> Output)

"1234"   -->  true
"12345"  -->  false
"a234"   -->  false

在线地址：Regex validate PIN code

function validatePIN(pin){
    return (pin.length === 4 || pin.length === 6) && /^\d+$/.test(pin)
}