Hexspeak - Practice Coding Problems

Hexspeak - Problem

Math String Easy

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".

Input & Output

Example 1 — Valid Hexspeak

$ Input: num = "257"

› Output: "IOI"

💡 Note: 257 in hex is 101. Replace '1'→'I' and '0'→'O' gives 'IOI'. All characters {I,O} are valid hexspeak.

Example 2 — Invalid Hexspeak

$ Input: num = "3"

› Output: "ERROR"

💡 Note: 3 in hex is 3. The digit '3' cannot be converted to valid hexspeak (not in {A,B,C,D,E,F,I,O}).

Example 3 — Large Valid Number

$ Input: num = "171"

› Output: "AB"

💡 Note: 171 in hex is AB. Both 'A' and 'B' are already valid hexspeak characters.

Constraints

1 ≤ num ≤ 10¹²
num consists of only digits

Visualization

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Understanding the Visualization

Input

Decimal number as string

Convert

Transform to hex and apply hexspeak rules

Validate

Check if result contains only valid characters

Key Takeaway

🎯 Key Insight: Only specific hex digits (0,1,A-F) can form valid hexspeak after transformation

Asked in

a Amazon 15 M Microsoft 8

The key insight is that only hex digits {0,1,A,B,C,D,E,F} can form valid hexspeak. Convert decimal to hex, replace 0→O and 1→I, then validate. Best approach is direct conversion with validation. Time: O(log n), Space: O(log n).

Common Approaches

Approach	Time	Space	Notes
✓ Direct Conversion and Validation	O(log n)	O(log n)	Convert to hex, replace characters, validate result
Single Pass Validation	O(log n)	O(log n)	Validate while converting to avoid extra passes

Direct Conversion and Validation — Algorithm Steps

Convert decimal to uppercase hex
Replace '0'→'O' and '1'→'I'
Check if all characters are in valid set

Visualization

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Step-by-Step Walkthrough

Convert to Hex

Convert decimal 257 to hex: 101

Replace Digits

Replace '0'→'O', '1'→'I': IOI

Validate

Check all chars in valid set: SUCCESS

Code -

solution.c — C

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

char* solution(char* num) {
    long long n = atoll(num);
    static char hexStr[32];
    sprintf(hexStr, "%llX", n);
    
    // Replace 0 with O and 1 with I
    for (int i = 0; hexStr[i]; i++) {
        if (hexStr[i] == '0') hexStr[i] = 'O';
        else if (hexStr[i] == '1') hexStr[i] = 'I';
    }
    
    // Check if all characters are valid
    for (int i = 0; hexStr[i]; i++) {
        char c = hexStr[i];
        if (c != 'A' && c != 'B' && c != 'C' && c != 'D' && 
            c != 'E' && c != 'F' && c != 'I' && c != 'O') {
            static char error[] = "ERROR";
            return error;
        }
    }
    
    return hexStr;
}

int main() {
    char num[32];
    fgets(num, sizeof(num), stdin);
    num[strcspn(num, "\n")] = 0;
    
    char* result = solution(num);
    printf("%s\n", result);
    return 0;
}

Time & Space Complexity

Time Complexity

⏱️

O(log n)

Converting to hex takes O(log n) digits, string operations are O(log n)

⚡ Linearithmic

Space Complexity

O(log n)

Storing hex string requires O(log n) space

⚡ Linearithmic Space

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Smart Actions

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Input & Output

Constraints

Visualization

Related Problems

Common Approaches

Direct Conversion and Validation — Algorithm Steps

Visualization

Code -

Time & Space Complexity

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