随意两个大整数的加减算法。可自己主动推断正负号。代码例如以下：

#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;

}