整数大数减法

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

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

int main(void)
{
int isSwap = 0;
char *a = "9988798794654987657946579849874654132156798794654984613246";
char *b = "5826143213410346162471303216654677941012324646321446546879";

char *tmp_a = (char *)malloc( sizeof(char) * strlen(a) + 1);
char *tmp_b = (char *)malloc( sizeof(char) * strlen(b) + 1);

if ( tmp_a==NULL || tmp_b==NULL )
{
//printf("分配内存tmp_a, tmp_b失败在文件：%s中第%d行\n", __FILE__, __LINE__);
exit(1);
}
else
{
strcpy(tmp_a, a);
strcpy(tmp_b, b);
//printf("分配tmp_a, tmp_b内存成功在文件：%s中第%d行\n", __FILE__, __LINE__);
}

char * tmp = NULL;

/*
确保a>b，用大数减小数，然后根据原来的数的大小来确定正负号
如果a<b则交换两个数
*/
if ( strlen(a) < strlen(b) || strcmp(a, b)<0)
{
tmp = (char *)malloc( sizeof(char) * (strlen(a)+1) );
if ( tmp== NULL )
{
//printf("分配内存tmp失败在文件：%s中第%d行\n", __FILE__, __LINE__);
exit(1);
}
else
{
// printf("分配内存tmp成功在文件：%s中第%d行\n", __FILE__, __LINE__);
}

strcpy(tmp, a);
// printf("tmp is %s\n", tmp);

a = (char *)malloc( sizeof(char) * (strlen(b)+1) );
memset(a, 0, sizeof(char) * strlen(a)+1);
strcpy(a, b);
// printf("a is %s\n", a);

b = (char *)malloc( sizeof(char) * (strlen(tmp)+1) );
memset(b, 0, sizeof(char) * strlen(b)+1);

strcpy(b, tmp);
//printf("b is %s\n", b);
isSwap = 1;
}
//printf("a is %s, b is %s\n", a, b);
char * result = BigNumberSubtration(a, b);
if ( result!=NULL)
{
if ( isSwap )
{
printf("  %-s\n -%-s \n =-%s \n", tmp_a, tmp_b, result);
}
else
{
printf("  %-s\n -%-s \n =%s \n", tmp_a, tmp_b, result);
}
}
else
{
printf("result is NULL!\n");
}
printf("\n");

free(tmp);
free(tmp_a);
free(tmp_b);
free(result);

tmp = NULL;
tmp_a = NULL;
tmp_b = NULL;
result = NULL;

system("pause");
return 0;
}

char * BigNumberSubtration(const char *a, const char *b)
{
int i = 0;
int j = 0;
int lengthOfA = strlen(a);
int lengthOfB = strlen(b);
int a_index = lengthOfA - 1;
int b_index = lengthOfB - 1;

char memberOfA[2];
char memberOfB[2];

int maxLength = (  lengthOfA>lengthOfB ? lengthOfA: lengthOfB ) + 1;

int * result = (int *)malloc( sizeof(int) *  maxLength );
char * char_result = (char *)malloc( sizeof(char) * maxLength );

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

memset(result, 0, sizeof(int) *  maxLength );
memset(char_result, 0, sizeof(char) *  maxLength );

for (i=maxLength-1; i>=0; i--)
{
if ( a_index >= 0 )
{
sprintf(memberOfA, "%c", a[a_index--]);
}
else
{
sprintf(memberOfA, "0");
}

if ( b_index >= 0 )
{
sprintf(memberOfB, "%c", b[b_index--]);
}
else
{
sprintf(memberOfB, "0");
}

result[i] = atoi(memberOfA) - atoi(memberOfB);
}
/*
for (i=0; i<maxLength; i++)
{
printf("%d ", result[i]);
}
printf("\n");
*/
for(i=maxLength-1; i>1; i--)
{
if ( result[i] < 0 )
{
result[i-1] = result[i-1] - 1;
result[i] = result[i] + 10;
}
}
/*
for (i=0; i<maxLength; i++)
{
printf("%d ", result[i]);
}
*/
i = 0;
while ( result[i]==0 )
{
i++;
}

for (j=0; i<maxLength; i++, j++)
{
char_result[j] = result[i] + '0';
}
char_result[j] = '\0';
printf("\n");
free(result);
result = NULL;
return char_result;
}