大数相乘——自己仿写版

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

char * BigNumberMulti(const char * a, const char *b);

int main(void)
{
	const char * a = "99999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999";
	const char * b = "9999999999999999999999999999999999999999";
	char * s = BigNumberMulti(a, b);
	if ( s==NULL )
	{
		printf("结果为NULL\n");
		exit(1);
	}
	else
	{
		printf("%s × %s = %s\n", a, b, s);
	}
	//free ( s );
	exit(EXIT_SUCCESS);
}

char * BigNumberMulti(const char * a, const char *b)
{
	int lengthOfA = strlen(a);
	int lengthOfB = strlen(b);

	int i = 0;
	int j = 0;

	int * result = (int *)malloc( sizeof(int) * (lengthOfA+lengthOfB) );
	char * char_result = (char * )malloc( sizeof(char) * (lengthOfA+lengthOfB) );

	if ( result==NULL || char_result==NULL)
	{
		printf("分配内存失败，在文件%s中第%d行!!\n", __FILE__, __LINE__);
		return NULL;
	}
	else
	{
		printf("分配内存成功，在文件%s中第%d行!!\n", __FILE__, __LINE__);
	}


	memset( result, 0, sizeof(int)*(lengthOfA+lengthOfB) );
	memset( char_result, 0, sizeof(char)*(lengthOfA+lengthOfB) );


	for (i=0; i<lengthOfA; i++)
	{
		for (j=0; j<lengthOfB; j++)
		{
			result[i+j+1] += ( a[i]-'0' ) * ( b[j]-'0' );
			//printf("s[%d+%d+1] is %d\n", i, j, result[i+j+1]);
		}
	}
	/*
	for (i=0; i<lengthOfA+lengthOfB; i++)
	{
		printf("%d ", result[i]);
	}
	*/

	for (i=lengthOfA+lengthOfB-1; i>=0; i--)
	{
		if ( result[i] >= 10 )
		{
			result[i-1] += result[i] / 10;
			result[i] = result[i] % 10;
		}
	}
	

	i=0;
	while( result[i] == 0 )
	{
		i++;
	}

	for (j=0; i<lengthOfA+lengthOfB; j++, i++)
	{
		char_result[j] = result[i]+'0';
	}
	char_result[j] = '\0';
	//printf("char_result is %s\n", char_result);
	free( result );
	result = NULL;
	return char_result;

}