哪位大佬能帮忙看看高精度加减乘除模是否有问题

#include <iostream>
#include <algorithm>
#include <cstdio>
#include <string>

using namespace std;

string clearZeros(string data)
{
	if (data[0] == '0') {
		int key = (int) data.length() - 1;
		for (int i = 0; i < data.length(); i++) {
	 	if (data[i] != '0') {
				key = i;
	            break;
	    	}
	    }
		data.erase(0, key);
	}
	if (data == "") {
		data = "0";
	}
    return data;
}

void countPoint(string &operand1, string &operand2)
{
    while (operand1.length() < operand2.length()) {
        operand1 = "0" + operand1;
    }
    while (operand1.length() > operand2.length()) {
        operand2 = "0" + operand2;
    }
}

bool bigger(string operand1, string operand2)
{
    return operand1 >= operand2;
}

string addition(string addent, string adder)
 {
    //先对位，在加数和被加数前面适当补0，使他们包含相同的位数
    countPoint(addent, adder);
    //前面再补一个0，确定和的最多位数
    addent = "0" + addent;
    adder = "0" + adder;
    //从低位开始，对应位相加，结果写进被加数中，如果有进位，直接给被加数前一位加1
    for (int i = (int) addent.length() - 1; i > 0; i--) {
        addent[i] = addent[i] + adder[i] - 48;
        if (addent[i] > '9') {
            addent[i] = addent[i] - 10;
            addent[i - 1] = addent[i - 1] + 1;
        }
    }
    return clearZeros(addent);
}

string subtraction(string subtrahend, string subtractor)
{
    //先对位，在减数和被减数前面适当补0，使他们包含相同的位数
    countPoint(subtrahend, subtractor);
    //判断被减数和减数谁大，保证被减数大于减数
    if (bigger(subtrahend, subtractor)) {
        subtrahend[0] = subtrahend[0] - subtractor[0] + 48;
        for (int i = 1; i < (int)subtrahend.length(); i++) {
            if (subtrahend[i] >= subtractor[i]) {
                subtrahend[i] = subtrahend[i] - subtractor[i] + 48;
            } else {
                subtrahend[i] = subtrahend[i] - subtractor[i] + 10 + 48;
                subtrahend[i - 1]--;
            }
        }
    } else {
        subtrahend = '-' + subtraction(subtractor, subtrahend);
    }
    return subtrahend;
}

string multiplication(string multiplicand, string multiplier)
{
     string result = "0";
    for (int i = (int)multiplier.length() - 1; i >= 0 ; i--) {
        for (char c = '1'; c <= multiplier[i]; c++) {
            result = addition(result, multiplicand);
        }
        multiplicand = multiplicand + "0";
    }
    return clearZeros(result);
}

string division(string dividend, string divisor, bool type = 0)
{
    string result;
    string resmod;
    for (register int i = 0; i < divisor.size() - 1; ++i) resmod += dividend[i];
    for (register int i = divisor.size() - 1; i < dividend.size() ; ++i)
    {
    	resmod += dividend[i];
    	result += '0';
    	while (subtraction(resmod, divisor) >= "0")
    	{
    		++result[result.size() - 1];
    		resmod = subtraction(resmod, divisor);
    	}
    	result = clearZeros(result);
    	resmod = clearZeros(resmod);
    }
    if (!type) return result;
    else return resmod;
}

string mod(string dividend, string modster)
{
	return division(dividend, modster, true);
}

string gcd(string a, string b)
{
	if (b == "0") return a;
	else return gcd(b, mod(a, b));
}

int main()
{
	string a, b, c;
	cin >> a >> b >> c;
	cout << gcd(addition(a, c), b);


	return 0;

}