Hexspeak

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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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

✓ Direct Conversion and Validation

⏱️ Time: O(log n) Space: O(log n)

Convert the decimal number to uppercase hexadecimal, replace '0' with 'O' and '1' with 'I', then check if all characters are in the valid hexspeak set.

Single Pass Validation

⏱️ Time: O(log n) Space: O(log n)

Build the hexspeak string character by character while simultaneously checking validity, returning early if any invalid hex digit is found.

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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Input & Output

Constraints

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Related Problems

Common Approaches

Direct Conversion and Validation — Algorithm Steps

Visualization

Code -

Time & Space Complexity

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