C_PRG_LANG_EX_3.4

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

void numb_shift(char *str)
{
    int len = strlen(str);
    str[0] = '1';

    for(int i = 1; i <= len; i++)
        str[i] = '0';

    str[len + 1] = '\0';

    return;
}

void sign_append(char *str)
{
    char cpy[256];
    cpy[0] = '-';
    for(int i = 1, j = 0; (cpy[i] = str[j]); i++, j++)
        ;

    return;
}

void my_itoa(int numb, char *str)
{
    int pos = 0;
    int sign = numb < 0 ? 1 : 0;

    if (numb < 0)
    {
        if (numb == INT_MIN)
            numb = INT_MAX;
        else
            numb = -(numb + 1);
    }

    while (numb)
    {
        str[pos++] = (numb % 10) + '0';
        numb /= 10;
    }
    str[pos] = '\0';

    int st_len = strlen(str);
    for(int i = st_len - 1, j = 0; i > j; i--, j++)
    {
        char tmp = str[i];
        str[i] = str[j];
        str[j] = tmp;
    }

    if (!sign)
        return;

    int flg_transfer = 0;  // Number transfer (69 + 1, etc.)
    for (pos = st_len - 1; pos >= 0; pos--)
    {
        flg_transfer = 0;
        if (str[pos] == '9')
        {
            str[pos] = '0';
            flg_transfer++;
        }
        else
        {
            str[pos] += 1;
            break;
        }
    }

    if (pos < 0)
        numb_shift(str);

    sign_append(str);

    return;
}

int main(void)
{
    int numb;
    char str_n[256];

    numb = INT_MAX;
    my_itoa(numb, str_n);
    printf("%s\n", str_n);

    numb = INT_MIN;
    my_itoa(numb, str_n);
    printf("%s\n", str_n);
}