大数相乘2

算法描述：

            9     8     7

*                 8     7

---

            63    56    49

      72    64    56             (第一个for循环中的累乘和累加)

---

      72    127   112  49

      72    127   116  9    

      72    138   6    9

      85    8     6    9                        (第二个for循环中的循环进位)

//大数相乘
#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <string.h>
#include <malloc.h>

void multiply(int *num1, int length1, int *num2, int length2, int *product)
{
	int i, j;
	for (j = length2 - 1; j >= 0; j--)
	{
		for (i = length1 - 1; i >= 0; i--)
		{
			product[i + j] += num1[i] * num2[j];
		}
	}

	for (i = length1 + length2 - 2; i >=1; i--)
	{
		if (product[i] > 10)
		{
			product[i - 1] += product[i] / 10;
			product[i] = product[i] % 10;
		}
	}
}

int main()
{
	char str1[255], str2[255];
	int length1, length2;
	int *num1, *num2;
	int *product;
	int i;

	gets(str1);
	gets(str2);

	length1 = strlen(str1);
	length2 = strlen(str2);

	num1 = malloc(sizeof(int)*length1);
	num2 = malloc(sizeof(int)*length2);
	product = malloc(sizeof(int)*(length1+length2-1));
	memset(product, 0, sizeof(int)*(length1 + length2 - 1));

	for (i = 0; i < length1; i++)
	{
		num1[i] = str1[i] - '0';
	}
	for (i = 0; i < length2; i++)
	{
		num2[i] = str2[i] - '0';
	}

	multiply(num1, length1, num2, length2, product);

	for (i = 0; i < length1+length2-1; i++)
	{
		printf("%d",product[i]);
	}
	return 0;
}