// C program to multiply two numbers represented
// as strings.
#include <stdio.h>
#include <string.h>
#include <stdlib.h>

// Multiplies str1 and str2, and prints result.
char* multiply(char* num1, char* num2)
{
    int n1 = strlen(num1);
    int n2 = strlen(num2);

    if (n1 == 0 || n2 == 0)
        return "0";

    // will keep the result number in vector
    // in reverse order
    int* result = malloc((n1+n2) * sizeof(int));
    for (int k = 0; k < (n1+n2); ++k){
        result[k] = 0;
    }

    // Below two indexes are used to find positions
    // in result.
    int i_n1 = 0;
    int i_n2 = 0;

    // Go from right to left in num1
    for (int i=n1-1; i>=0; i--)
    {
        int carry = 0;
        int n1 = num1[i] - '0';

        // To shift position to left after every
        // multiplication of a digit in num2
        i_n2 = 0;

        // Go from right to left in num2
        for (int j=n2-1; j>=0; j--)
        {
            // Take current digit of second number
            int n2 = num2[j] - '0';

            // Multiply with current digit of first number
            // and add result to previously stored result
            // at current position.
            int sum = n1*n2 + result[i_n1 + i_n2] + carry;

            // Carry for next iteration
            carry = sum/10;

            // Store result
            result[i_n1 + i_n2] = sum % 10;

            i_n2++;
        }

        // store carry in next cell
        if (carry > 0)
            result[i_n1 + i_n2] += carry;

        // To shift position to left after every
        // multiplication of a digit in num1.
        i_n1++;
    }

    // ignore '0's from the right
    int i = (n1+n2) - 1;
    while (i>=0 && result[i] == 0)
        i--;

    // If all were '0's - means either both or
    // one of num1 or num2 were '0'
    if (i == -1)
        return "0";

    // generate the result string
    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;
}

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

    printf("%s\n",multiply(str1, str2));
    return 0;
}