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

char* multiply(char* str1, char* str2)
{
    int lb1 = strlen(str1);
    int lb2 = strlen(str2);

    if (lb1 == 0 || lb2 == 0)
        return "0";

    int* result = malloc((lb1+lb2) * sizeof(int));
    for (int k = 0; k < (lb1+lb2); ++k){
        result[k] = 0;
    }

    int i_lb1 = 0;
    int i_lb2 = 0;

    for (int i=lb1-1; i>=0; i--)
    {
        int trzym = 0;
        int lb1 = str1[i] - '0';

        i_lb2 = 0;

        for (int j=lb2-1; j>=0; j--)
        {
            int lb2 = str2[j] - '0';

            int sum = lb1*lb2 + result[i_lb1 + i_lb2] + trzym;

            trzym = sum/10;

            result[i_lb1 + i_lb2] = sum % 10;

            i_lb2++;
        }

        if (trzym > 0)
            result[i_lb1 + i_lb2] += trzym;

        i_lb1++;
    }

    int i = (lb1+lb2) - 1;
    while (i>=0 && result[i] == 0)
        i--;

    if (i == -1)
        return "0";

    char* s;
    s = malloc(i*sizeof(char));
    s[0] = '\0';

    char *buffer = malloc(sizeof(char));
    buffer[0] = '\0';
    while (i >= 0){
        sprintf(buffer,"%d",result[i--]);
        strcat(s,buffer);
    };

    free(buffer);
    free(result);

    return s;
}

int main()
{
    char* str1 = "12";
    char* str2 = "12";

    printf("Wynik mnozenia dwoch liczb: ");
    printf("%s\n",multiply(str1, str2));
    return 0;
}