my_itoa.c

#include <stdio.h>

#define DIGITS "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ"

void my_itoa(int num, char* str, int base) {
    if (num < 0) {
        num *= -1;
        str[0] = '-';
        str++;
    }

    int base_power = 1;
    int num_digits = 1;
    while (num / base_power > base - 1) {
        base_power *= base;
        num_digits++;
    }

    int i = 0;
    for (; i < num_digits; i++) {
        int digit = (num / base_power) % base;
        str[i] = DIGITS[digit];
        base_power /= base;
    }
    str[i] = '\0';
}

int main() {
    char word[100];
    for (int i = -500; i < 501; i += 5) {
        my_itoa(i, word, 10);
        printf("%s\n", word);
        my_itoa(i, word, 16);
        printf("%s\n", word);
    }
}