a+b and a-b

a+b:

#include<bits/stdc++.h>
using namespace std;
char s1[505],s2[505];
int a[505],b[505],c[505];
int main(){
	int la,lb,lc;
	scanf("%s",s1);
	scanf("%s",s2);
	la=strlen(s1);  
	lb=strlen(s2);	
	for(int i=0;i<la;i++)
	{
		a[la-i]=s1[i]-'0';	
	}
	for(int i=0;i<lb;i++)
	{
		b[lb-i]=s2[i]-'0';
	} 
	lc=max(la,lb)+1;
	for(int i=1;i<=lc;i++)
	{
		c[i]+=a[i]+b[i];
		c[i+1]=c[i]/10;	 
		c[i]=c[i]%10;	
	} 
	if(c[lc]==0 && lc>0) lc--; 
	for(int i=lc;i>0;i--)
		cout<<c[i]; 	
	return 0; 
 } 

a-b:

#include<bits/stdc++.h>
using namespace std; 
const int maxn = 10010; 
int len1, len2; 
int a[maxn], b[maxn]; 
string s1, s2; 
void swap(string &a, string &b){
    string t; 
    t = a; 
    a = b; 
    b = t; 
}
bool compare(string a, string b){
    if (a.size() < b.size()) return true;  //当a的位数比b小时，a比b小 
    if (a.size() > b.size()) return false; //当a的位数比b大时，a比b大
    for (int i = 0; i < a.size(); ++i)     //剩余情况就是a的位数与b相同 
    {
        if (a[i] > b[i]) return false;     //逐位比较 
        if (b[i] > a[i]) return true; 
    }
    return false; //最后当a与b的各个位的数字都相同时，a等于b 
}
int main()
{
    cin >> s1;           
    cin >> s2; 
    if (compare(s1, s2)){
        swap(s1, s2);  
        putchar('-'); 
    }
    len1 = s1.size(); 
    len2 = s2.size(); 
    for (int i = 0; i < len1; ++i) a[len1 - i] = s1[i] - '0'; 
    for (int i = 0; i < len2; ++i) b[len2 - i] = s2[i] - '0'; 
    for (int i = 1; i <= len1; ++i){
        a[i] -= b[i];  
        if (a[i] < 0) {                
            a[i + 1]--;             //就向高位借一 
            a[i] += 10; 
        }
    }
    while (a[len1] == 0 && len1 > 1) len1--; 
    for (int i = len1; i >= 1; --i) printf("%d", a[i]); 
    return 0; 
}