isPalindrome("") = true
isPalindrome("A") = true
isPalindrome("AA") = true
isPalindrome("AB") = false
isPalindrome("AAA") = true
isPalindrome("AAB") = false
isPalindrome("ABA") = true
isPalindrome("ABC") = false
isPalindrome("AAAA") = true
isPalindrome("AAAB") = false
isPalindrome("AABC") = false
isPalindrome("ABBA") = true
isPalindrome("ABBC") = false
isPalindrome("ABCA") = false
isPalindrome("ABCD") = false