C语言：大数相加与大数相减.

??#include<stdio.h>
#include<string.h>
#define N 100
int res[N] = {0};

char *add(char a[],int n,char b[],int m) //大数相加
{
	int i=N-1,z=0,j;
	for( ; n>=0; n--,m--)
	{
		if(m>=0)
			res[i] = res[i] + a[n]-'0' + b[m]-'0';//res[i]=0或res[i]=1;
		else
			res[i] = res[i] + a[n] - '0';
		if( res[i] > 9 )
		{
			res[i] = res[i] - 10;
		    res[i-1]++;
		} 
		i--;
	}

	for(j=0 ;i<=N-1; i++,j++)
		a[j] = res[i] + '0';
	a[j]='';

	while(a[z] == '0')
		z++;
		a = a+z;
	return a;
}
char *change(char a[],int m)
{
	if(a[0]=='0' && b[0] == '0')
	return "0";

	else 
		if(n>=m)
		a = add(a,n,b,m);
	else
		a = add(b,m,a,n);
	return a;
}
void main()
{
	char a[N],b[N],*s;
	int n,m;
	printf("Please input the first number: ");
	gets(a);
	printf("nnPlease input the second number: ");
	fflush(stdin);
	gets(b);

	n = strlen(a)-1;
	m = strlen(b)-1;

	printf("nnThe first number add the second number is: ");
	s = change(a,m);
	puts(s);
}


#include<stdio.h>
#include<string.h>
char *sub(char a[],int m) //大数相减
{
	int z=0;
	for( ; n>=0; n--,m--)
	{
		if(m>=0)
			a[n] = a[n] - b[m] + '0';
		else
			a[n] =  a[n];
		if(a[n] < '0')
		{
			a[n] = a[n] +10;
			a[n-1]--;
		}
	}
	while(a[z] == '0')
			z++;
		a = a+z;
		return a;
}
char *change(char a[],int m)
{
	if(n>m)
		a = sub(a,m);
	if(n<m)
	{
		printf("-");
		a = sub(b,n);
	}
	if(n==m)
	{
		if(strcmp(a,b) > 0)
			a = sub(a,m);
		else
			if(strcmp(a,b) <0)
		{
			printf("-");
		a = sub(b,n);
		}
			else
				return "0";
	}
	return a;
}
void main()
{
	char a[100],b[100],m;
	printf("请输入被减数: ");
	gets(a);
	printf("nn请输入减数: ");
	fflush(stdin);
	gets(b);
	n = strlen(a)-1;
	m = strlen(b)-1;
	printf("nn两数相减: ");
	s = change(a,m);	
	puts(s);
	printf("n");
}