Formatted question description:

1271. Hexspeak




A decimal number can be converted to its Hexspeak representation by first converting it to an uppercase hexadecimal string, then replacing all occurrences of the digit 0 with the letter O, and the digit 1 with the letter I. Such a representation is valid if and only if it consists only of the letters in the set {"A", "B", "C", "D", "E", "F", "I", "O"}.

Given a string num representing a decimal integer N, return the Hexspeak representation of N if it is valid, otherwise return "ERROR".

Example 1:

Input: num = “257”

Output: “IOI”

Explanation: 257 is 101 in hexadecimal.

Example 2:

Input: num = “3”

Output: “ERROR”


  • 1 <= N <= 10^12
  • There are no leading zeros in the given string.
  • All answers must be in uppercase letters.


The solution is quite straightforward. Convert the decimal number to the hexadecimal string, change the string to uppercase, and replace 0 with O and 1 with I. Then check each character in the string. If a character out of the set {"A", "B", "C", "D", "E", "F", "I", "O"} occurs, return "ERROR". If the string is valid, return the string.

class Solution {
    public String toHexspeak(String num) {
        long decimal = Long.parseLong(num);
        String hex = toHex(decimal);
        hex = hex.toUpperCase();
        hex = hex.replaceAll("0", "O");
        hex = hex.replaceAll("1", "I");
        String validStr = "ABCDEFIO";
        char[] hexArray = hex.toCharArray();
        for (char c : hexArray) {
            if (validStr.indexOf(c) < 0)
                return "ERROR";
        return hex;

    public String toHex(long num) {
        String hex = "";
        while (num > 0) {
            int remainder = (int) (num % 16);
            String digit = remainder < 10 ? String.valueOf(remainder) : String.valueOf((char) ('A' + (remainder - 10)));
            hex = digit + hex;
            num /= 16;
        return hex;

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