Hexspeak

Hexspeak

What is this problem about?

The Hexspeak interview question is a playful take on hexadecimal numbers. In "Hexspeak," only the letters "A", "B", "C", "D", "E", "F" and the digits "1" and "0" are allowed. You are given a decimal number as a string. You must convert it to hexadecimal. If the hexadecimal representation contains any digits from "2" to "9", it's not a valid Hexspeak word, and you should return "ERROR". Otherwise, you replace "1" with "I" and "0" with "O" and return the resulting string.

Why is this asked in interviews?

This "Easy" level question tests string formatting, base conversion, and conditional logic. Firms like Virtu Financial use it to check a candidate's attention to specific business rules and their ability to perform multi-step data transformations. It evaluations if you can correctly identify valid/invalid output states based on a custom encoding scheme.

Algorithmic pattern used

The problem uses a Math and Simulation pattern.

1. Convert the input string (decimal) to a long integer.
2. Convert that integer to its hexadecimal string representation.
3. Iterate through the hex string:
   • If a digit is between 2 and 9, return "ERROR".
   • If a digit is '1', append 'I'.
   • If a digit is '0', append 'O'.
   • If a character is 'A'-'F', append it as is.

Example explanation

Input: "257"

1. Convert to hex: $257_{10} = 101_{16}$.
2. Check digits: '1', '0', '1'. All are valid for Hexspeak.
3. Replace: '1' $o$ 'I', '0' $o$ 'O', '1' $o$ 'I'. Result: "IOI".

Input: "3"

1. Convert to hex: $3_{10} = 3_{16}$.
2. Check digit '3': Not valid (2-9). Result: "ERROR".

Common mistakes candidates make

• Case Sensitivity: Producing lowercase letters when the problem requires uppercase.
• Integer Overflow: Using a standard 32-bit integer for the input string, which can represent numbers up to $10^{12}$ (requiring Long or BigInt).
• Wrong replacement: Swapping 'I' for '0' or 'O' for '1' by mistake.

Interview preparation tip

Practice using built-in language functions for hex conversion (like Long.toHexString() in Java or hex() in Python). However, be ready to implement the conversion manually using the divide-by-16 method if the interviewer asks.