Q10106: Product

/*
    大數的乘法
    必須利用int arr[n]去一個一個乘
    這跟階層到很高的階層數的題目類似

    Run time: 0.000
*/
#include <stdio.h>
#include <string.h>
#define N 251
char prod1[N];
char prod2[N];
int arr1[N];
int arr2[N];
int ans[2*N];
void to_INT(int len1, int len2);
void prod(int len1, int len2);

int main()
{
    int len1, len2;
    while (gets(prod1) != NULL)
    {
        gets(prod2);

        len1 = strlen(prod1);
        len2 = strlen(prod2);

        to_INT(len1, len2);

        prod(len1, len2);

    }

    return 0;
}

void to_INT(int len1, int len2)
{
    int i, j;

    for ( i = len1-1, j = 0; i >= 0; i--, j++)
        arr1[j] = prod1[i]-'0';

    for ( i = len2-1, j = 0; i >= 0; i--, j++)
        arr2[j] = prod2[i]-'0';
}

void prod(int len1, int len2)
{
    int i, j, k;

    for ( i = 0; i < 2*N; i++)
        ans[i] = 0;

    for ( i = 0; i < len1; i++)
    {
        for ( j = 0, k = i; j < len2; j++)
            ans[k++] += arr1[i]*arr2[j];

        for ( j = 0; j < k-1; j++)
        {
            if (ans[j] >= 10)
            {
                ans[j+1] += ans[j]/10;
                ans[j] %= 10;
            }
        }
    }

    for ( i = k-1; i >= 0; i--)
        if (ans[i]) break;

    if (i == -1)  printf("0\n");

    else
    {
        for ( ; i >= 0; i--)
            printf("%d", ans[i]);
        printf("\n");
    }

}