Seperate small letters and capped letters

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

void small_cap(char* str, char** str_small, char** str_cap);

int main()
{
    char str[] = "AaBb1231_Cc_)!@#B";

    char* str_small, * str_cap;

    small_cap(str, &str_small, &str_cap);

    printf("\nThe string is: %s\n", str);

    if (str_small == NULL || str_cap == NULL)                   // check null
    {
        printf("allocation failed");
        return 1;
    }

    printf("\nCap: %s\nSmall: %s\n\n", str_cap, str_small);

    free(str_small);
    free(str_cap);

    return 0;
}

void small_cap(char* str, char** str_small, char** str_cap)
{
    int len = strlen(str);
    int countC = 0, countS = 0, j = 0, z = 0;

    for (int i = 0; i < len; i++)
    {
        if (str[i] >= 'a' && str[i] <= 'z')
            countS++;

        else if (str[i] >= 'A' && str[i] <= 'Z')
            countC++;
    }

    *str_small = (char*)malloc(1 + countS * sizeof(char));
    *str_cap = (char*)malloc(1 + countC * sizeof(char));

    if (*str_small && *str_cap)
    {
        for (int i = 0; i < len; i++)
        {
            if (str[i] >= 'a' && str[i] <= 'z')
                (*str_small)[j++] = str[i];

            else if (str[i] >= 'A' && str[i] <= 'Z')
                (*str_cap)[z++] = str[i];
        }

        (*str_small)[j] = '\0';
        (*str_cap)[z] = '\0';
    }
}