【C++】已经通过

#include<string>
#include<algorithm>
using namespace std;
//小于返回true;大于或者等于返回false
bool isless(string a, string b) {
	if (a.size() > b.size()) {
		return false;
	}
	else if (a.size() == b.size()) {
		for (int i = 0; i < a.size(); i++) {
			if (a[i] != b[i]) {
				return a[i] < b[i];
			}
		}
		return false;
	}
	else {
		return true;
	}
}
string stradd(string a, string b) {
	string ls, ss;
	if (a.size() > b.size()) {
		ls = a; ss = b;
	}
	else {
		ls = b; ss = a;
	}
	if (ls.size() == 0)return "0";
	if (ss.size() == 0)ss = "0";
	reverse(ls.begin(), ls.end());
	reverse(ss.begin(), ss.end());

	int carry = 0; int i = 0;
	for (; i< ss.size(); i++) {
		if (ls[i] - '0' + ss[i] - '0' + carry <= 9) {
			ls[i] = (ls[i] - '0' + ss[i] - '0' + carry) + '0';
			carry = 0;
		}
		else {
			ls[i] = (ls[i] - '0' + ss[i] - '0' + carry - 10) + '0';
			carry = 1;
		}
	}
	if (i == ls.size()&&carry==1) {
		ls += "1";
	}
	else {
		while (carry == 1) {
			if (ls[i] - '0' + + carry <= 9) {
				ls[i] = (ls[i] - '0' +  carry) + '0';
				carry = 0;
			}
			else {
				ls[i] = (ls[i] - '0' +  carry - 10) + '0';
				carry = 1;
				if (i == ls.size() - 1) {
					ls += "1";
					carry = 0;
				}
			}
			i++;
		}
	}
	
	reverse(ls.begin(), ls.end());
	return ls;
}
//返回a-b,只考虑a>=b的情况
string strsub(string a, string b) {
	bool positive = true;//true表示正数，false表示结果为负
	
	string ls, ss;
	if (isless(a, b)) {
		positive = false;
		ls = b; ss = a;
	}
	else {
		positive = true;
		ls = a; ss = b;
	}
	if (ls.size() == 0)return "0";
	if (ss.size() == 0)ss = "0";
	reverse(ls.begin(), ls.end());
	reverse(ss.begin(), ss.end());
	
	int carry = 0; int i = 0;
	for (; i < ss.size(); i++) {
		if (ls[i] - ss[i] - carry < 0) {
			ls[i] = ls[i] - ss[i] - carry + 10 + '0';
			carry = 1;
		}
		else {
			ls[i] = ls[i] - ss[i] - carry + '0';
			carry = 0;
		}
	}
	for (; i < ls.size(); i++) {
		if (ls[i]  - '0'-carry < 0) {
			ls[i] = ls[i]  - carry + 10 ;
			carry = 1;
		}
		else {
			ls[i] = ls[i]  - carry ;
			carry = 0;
		}
	}

	reverse(ls.begin(), ls.end());
	while (ls[0] == '0') {
		//cout << "清除零" << endl;
		ls.erase(0, 1);
	}
	if (ls == "") {
		ls = "0";
	}
	if (positive) {

	}
	else {
		ls.insert(0, "-");
	}
	return ls;
}
string strmul(string a, string b) {
	string ls, ss;
	if (a.size() > b.size()) {
		ls = a; ss = b;
	}
	else {
		ls = b; ss = a;
	}
	if (ss.size() == 0)return "0";

	string result = "";
	for (int i = 0; i < ss.size(); i++) {
		string temp = "";
		for (int j = 0; j < ss[i]-'0'; j++) {
			temp = stradd(temp, ls);
		}
		result = stradd(result, temp);
		if (i != ss.size() - 1) {
			result += "0";
		}
	}

	/*reverse(ls.begin(), ls.end());
	reverse(ss.begin(), ss.end());*/
	return result;
}
void test() {
	string s1 = "16";
	string s2 = "4";
	cout << strsub(s1, s2) << endl;
	cout << "-----------------------" << endl;
}
int main() {
	//test();
	string a, b;
	cin >> a >> b;
	cout << stradd(a, b) << endl << strsub(a, b) << endl << strmul(a, b) << endl;

	return 0;
}