任意两个大整数的加减算法，可自动判断正负号，代码如下：

#include <iostream>
#include <vector>
#include <cstring>
#include <algorithm>
#include <string>

using namespace std;

string BigInegerAdd(string s1, string s2) // s1+s2;
{
	int len = s1.size()>s2.size()?s1.size()+1:s2.size()+1;
	string res(len, '0');
	int i = s1.size() - 1, j = s2.size() - 1, k = len - 1;
	int borrow = 0;
	while(i >=0 && j >= 0)
	{
		int sum = s1[i] - '0' + s2[j] - '0' + borrow;
		if (sum >= 10)
		{
			borrow = 1;
			sum -= 10;
			res[k--] = sum + '0';
		}
		else {res[k--] = sum + '0'; borrow = 0;}
		i--;j--;
	}
	while(i >= 0)
	{
		int sum = s1[i] - '0' + borrow;
		if (sum >= 10)
		{
			borrow = 1;
			sum -= 10;
			res[k--] = sum + '0';
		}
		else {res[k--] = sum + '0'; borrow = 0;}
		i--;
	}
	while(j >= 0)
	{
		int sum = s2[j] - '0' + borrow;
		if (sum >= 10)
		{
			borrow = 1;
			sum -= 10;
			res[k--] = sum + '0';
		}
		else {res[k--] = sum + '0'; borrow = 0;}
		j--;
	}
	if (borrow == 1)
	{
		res[k] = '1';
	}
	else res[k] = '0';
	if (res[0] == '0')//ignore the prefix '0's
	{
		return res.substr(1, res.size()-1);
	}
	else return res;

}

//negative == true means s1 < s2
string BigInegerMinus(string s1, string s2, bool negative) // s1-s2; 
{

	if (s1.size() < s2.size())
	{
		return BigInegerMinus(s2, s1, true);
	}
	if (s1.size() == s2.size())
	{
		int i = 0;
		while(i < s1.size() && s1[i] == s2[i])
			i++;
		if (s1[i] < s2[i])
		{
			return BigInegerMinus(s2, s1, true);
		}
	}
	string res(s1.size(), '0');
	int i = s1.size() -1, j = s2.size() - 1;
	int k = i;
	int borrow = 0;
	while(i >= 0 && j >= 0)
	{
		int sum = s1[i] - '0' - borrow - (s2[j] - '0');
		//cout<<sum<<endl;
		if (sum < 0)
		{
			borrow = 1;
			sum += 10;
			res[k--] = sum + '0';
		}
		else{
			borrow = 0;
			res[k--] = sum + '0';
		}
		i--;j--;
	}
	while(i >= 0)
	{
		int sum = s1[i--] - '0' - borrow;
		if (sum < 0)
		{
			borrow = 1;
			sum += 10;
			res[k--] = sum + '0';
		}
		else{
			borrow = 0;
			res[k--] = sum + '0';
		}
	}
	if (res[0] == '0')
	{
		//ignore the prefix '0's
		int index = 1;
		while(index < res.size() && res[index] == '0')
			index++;
		if (negative)
		{
			return "-" + res.substr(index, res.size() - index);
		}
		else return res.substr(index, res.size() - index);
	}
	else {
		if (negative)
		{
			return "-" + res;
		}
		else return res;
	}
}

string BigIneger(string s1, string s2)
{
	if (s1 == "")
	{
		return s2;
	}
	if (s2 == "")
	{
		return s1;
	}

	char sign1 = '+', sign2 = '+';
	if (s1[0] == '-')
	{ 
		sign1 = '-';
	}
	if (s2[0] == '-')
	{
		sign2 = '-';
	}
	if (sign1 == '+' && sign2 == '+')
	{
		return BigInegerAdd(s1, s2);
	}
	else if (sign1 == '+' && sign2 == '-')
	{
		return BigInegerMinus(s1, s2.substr(1, s2.size()-1),false);
	}
	else if (sign1 == '-' && sign2 == '+')
	{
		return BigInegerMinus(s2, s1.substr(1, s1.size()-1), false);
	}
	else {
		string tmp = BigInegerAdd(s1.substr(1, s1.size()-1), s2.substr(1, s2.size()-1));
		if (tmp[0] == '0')
		{
			tmp = tmp.substr(1, tmp.size()- 1);
		}
		string res = "-" + tmp;
		return res;
	}
}


int main()
{
	string s1 = "-789546321", s2 = "-15462897444";
	cout<<BigIneger(s1,s2)<<endl;
	return 0;
}