大数相加

string add(string n1,string n2){
	int len1 = n1.length(),len2 = n2.length(),carry = 0;
	string result;
	for(int i = len1-1,j = len2-1; i >= 0 || j >= 0; i--,j--){
		int sum = carry;
		if(i >= 0) sum += n1[i]-'0';
		if(j >= 0) sum += n2[j]-'0';
		carry = sum/10;
		sum = sum%10;
		result.push_back(char('0'+sum));
	}
	if(carry) result.push_back(char('0'+carry));
	reverse(result.begin(),result.end());
	return result;
}