大整数类（持续更新）

目前只重载了+、-、*运算符，支持正数的加法、减法、乘法运算。并可用插入符>>提取符<<进行输入输出
取模、除法、负数、gcd、大小判定（重载><=）、派生高精度浮点数等待更新

#include<iostream>
#include<string>
#include<algorithm> 
using namespace std;
inline int ctoi(char c){
    return c - '0';
}
inline char itoc(int i){
   	return i + '0';
}
class BigInteger{
    typedef long long ll;
	public:
    	BigInteger(ll a = 0LL){
    		if(a == 0)	s = "0";
        	while(a){
          		s.insert(s.begin(), itoc((int)a%10));
				a /= 10;
       		}
   		}
   		BigInteger(string s): s(s){  }
   		BigInteger add(const BigInteger& BigInt);
		BigInteger substract(const BigInteger& BigInt);
		BigInteger multiply(const BigInteger& BigInt);
   		friend ostream& operator <<(ostream& out, const BigInteger& BigInt); 
		friend istream& operator >>(istream& in, BigInteger& BigInt);
		friend BigInteger operator +(const BigInteger& BigInt1, const BigInteger& BigInt2);
		friend BigInteger operator -(const BigInteger& BigInt1, const BigInteger& BigInt2);
		friend BigInteger operator *(const BigInteger& BigInt1, const BigInteger& BigInt2);
	private:
    	string s;
};

ostream& operator <<(ostream& out, const BigInteger& BigInt){
	out << BigInt.s;
	return out;
}
istream& operator >>(istream& in, BigInteger& BigInt){
	in >> BigInt.s;
	return in;
}
BigInteger operator +(const BigInteger& BigInt1, const BigInteger& BigInt2){
	string a = BigInt1.s;
	string b = BigInt2.s;
	reverse(a.begin(), a.end()); reverse(b.begin(), b.end());
	if(a.length() < b.length()) swap(a, b);
	int carry = 0;
	for(int i = 0; i < (int)a.length(); i++){
		int temp = (i < (int)b.length()? ctoi(b[i]): 0) + ctoi(a[i]) + carry;
		carry = temp / 10;
		temp %= 10;
		a[i] = itoc(temp);
	}
	reverse(a.begin(), a.end());
	if(carry) a.insert(a.begin(), itoc(carry));
	return BigInteger(a);
}
BigInteger operator -(const BigInteger& BigInt1, const BigInteger& BigInt2){
	string a = BigInt1.s;
	string b = BigInt2.s;
	reverse(a.begin(), a.end()); reverse(b.begin(), b.end());
	int carry = 0;
	for(int i = 0; i < (int)a.length(); i++){
		int temp = ctoi(a[i]) - (i < (int)b.length()? ctoi(b[i]): 0) + carry;
		if(temp < 0) temp += 10,carry = -1;
		else carry = 0;
		a[i] = itoc(temp);
	}
	reverse(a.begin(), a.end());
	a.erase(0, a.find_first_not_of('0'));
	if(a == "")	a == "0"; 
	return BigInteger(a);
}
BigInteger operator *(const BigInteger& BigInt1, const BigInteger& BigInt2){
	string a = BigInt1.s;
	string b = BigInt2.s;
	reverse(a.begin(), a.end()); reverse(b.begin(), b.end());
	if(a.length() < b.length())	swap(a, b);
	BigInteger ans(0);
	for(int i = 0; i < (int)b.size(); i++){
		int carry = 0;
		string c = "";
		for(int j = 0; j < (int)a.size(); j++){
			int temp = ctoi(a[j]) * ctoi(b[i]) + carry;
			carry = temp / 10;
			temp %= 10;
			c.insert(c.begin(), itoc(temp));
		}
		if(carry) c.insert(c.begin(), itoc(carry));
		int cnt = i;
		while(cnt--)	c += '0';
		c.erase(0, c.find_first_not_of('0'));
		if(c == "")	c = "0";
		ans.add(c);	
	}
	return ans;
}
BigInteger BigInteger::add(const BigInteger& BigInt){
	*this = *this+ BigInt;
	return *this;
}
BigInteger BigInteger::substract(const BigInteger& BigInt){
	*this = *this - BigInt;
	return *this;
}
BigInteger BigInteger::multiply(const BigInteger& BigInt){
	*this = *this - BigInt;
	return *this;
}
int main(void){
	string s1, s2;
	while(cin >> s1 >> s2){
		BigInteger a(s1), b(s2);
		cout << a - b << endl;
	} 
	return 0;
}