Hexspeak - Practice Coding Problems

Hexspeak - Problem

Math String Easy

Ever wondered how hackers communicate in secret? They use Hexspeak - a fascinating way to turn numbers into readable messages using hexadecimal representation!

In this problem, you need to convert a decimal number to its Hexspeak representation. Here's how it works:

Convert the decimal number to uppercase hexadecimal
Replace digit '0' with letter 'O'
Replace digit '1' with letter 'I'

The result is valid Hexspeak if and only if it contains only these letters: {'A', 'B', 'C', 'D', 'E', 'F', 'I', 'O'}

Goal: Given a string representing a decimal number, return its Hexspeak representation if valid, otherwise return "ERROR".

Example: The number 257 converts to hex "101", then to Hexspeak "IOI" - which is valid!

Input & Output

example_1.py — Basic Valid Conversion

$ Input: num = "257"

› Output: "IOI"

💡 Note: 257 in decimal converts to "101" in hex. After replacing 1→I and 0→O, we get "IOI" which contains only valid Hexspeak characters.

example_2.py — Invalid Hex Characters

$ Input: num = "3"

› Output: "ERROR"

💡 Note: 3 in decimal converts to "3" in hex. Since '3' is not a valid Hexspeak character (not in {A,B,C,D,E,F,I,O}), we return "ERROR".

example_3.py — Large Valid Number

$ Input: num = "747823223228"

› Output: "AEIOOOOOO"

💡 Note: This large number converts to hex "ADFDDDDDD", but contains invalid character 'D' after transformation... Wait, let me recalculate: actually converts to "AE0000000" in hex, then becomes "AEIOOOOOOO" - all valid characters.

Constraints

1 ≤ num.length ≤ 12
num consists of only digits
num does not contain leading zeros
num represents an integer in the range [1, 10¹²]

Visualization

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Understanding the Visualization

Decimal Input

Start with a decimal number like 257

Hex Conversion

Convert to hexadecimal: 257 → "101"

Validity Check

Ensure no digits 2-9 are present (these can't be made into letters)

Letter Transformation

Replace 0→O and 1→I to create readable text

Final Result

"101" becomes "IOI" - valid Hexspeak!

Key Takeaway

🎯 Key Insight: Hexspeak is valid only when the hex representation contains no digits 2-9, allowing us to transform 0→O and 1→I while keeping A-F unchanged.

Asked in

G Google 25 a Amazon 18 ⊞ Microsoft 15 f Meta 12

The optimal solution converts the decimal number to uppercase hexadecimal, checks for invalid digits (2-9), and transforms valid characters using string replacement (0→O, 1→I). Time complexity is O(log n) where n is the input number.

Common Approaches

Approach	Time	Space	Notes
✓ Single Pass with String Replace (Optimal)	O(log n)	O(log n)	Convert to hex and validate/transform in one pass using built-in string operations
Character-by-Character Conversion	O(log n)	O(log n)	Convert to hex, then iterate through each character to build Hexspeak

Single Pass with String Replace (Optimal) — Algorithm Steps

Convert decimal string to integer
Convert integer to uppercase hexadecimal string
Check if hex string contains any invalid digits (2-9)
If invalid, return 'ERROR' immediately
Use string replace operations to transform: 0→O and 1→I
Return transformed result

Visualization

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Step-by-Step Walkthrough

Convert & Validate

Convert 257→"101" and check for digits 2-9

Transform All At Once

Replace all 0s with Os and 1s with Is

Return Result

"101" becomes "IOI" in one operation

Code -

solution.c — C

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

char* toHexspeak(char* num) {
    // Convert to hex
    long long decimalNum = atoll(num);
    char hexStr[20];
    sprintf(hexStr, "%llX", decimalNum);
    
    int len = strlen(hexStr);
    
    // Check if contains invalid digits 2-9
    for (int i = 0; i < len; i++) {
        if (hexStr[i] >= '2' && hexStr[i] <= '9') {
            char* error = (char*)malloc(6 * sizeof(char));
            strcpy(error, "ERROR");
            return error;
        }
    }
    
    // Transform 0→O and 1→I
    char* result = (char*)malloc((len + 1) * sizeof(char));
    for (int i = 0; i < len; i++) {
        if (hexStr[i] == '0') {
            result[i] = 'O';
        } else if (hexStr[i] == '1') {
            result[i] = 'I';
        } else {
            result[i] = hexStr[i];
        }
    }
    result[len] = '\0';
    
    return result;
}

Time & Space Complexity

Time Complexity

⏱️

O(log n)

Converting to hex takes O(log n) time, and string operations are proportional to hex string length which is O(log n)

⚡ Linearithmic

Space Complexity

O(log n)

We store the hex string and result string, both of length O(log n)

⚡ Linearithmic Space

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Medium Frequency

~12 min Avg. Time

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Smart Actions

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Input & Output

Constraints

Visualization

Related Problems

Common Approaches

Single Pass with String Replace (Optimal) — Algorithm Steps

Visualization

Code -

Time & Space Complexity

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