【C++】利用加法实现string的乘法

#include<string>
#include<algorithm>
using namespace std;
#define MAXINT 1000000000
//写一个字符串的乘法
//加法
string stradd(string a, string b) {
	if (a.size() == 0 && b.size() == 0) return "0";
	if (a.size() == 0)return b;
	if (b.size() == 0)return a;

	string ls, ss;
	if (a.size() >= b.size()) {
		ls = a; ss = b;
	}
	else {
		ls = b; ss = a;
	}
	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;
		}
	}
	while (carry != 0) {
		if (i == ls.size()) {
			carry = 0;
			ls += "1";
		}
		else {
			if (ls[i] - '0' + carry <= 9) {
				ls[i] = ls[i] + carry;
				carry = 0;
			}
			else {
				ls[i] = ls[i] + carry -10;
				carry = 1;
			}
		}
		i++;
	}

	reverse(ls.begin(), ls.end());
	return ls;
}
//将a乘以b，结果用string返回
string strmul(string a, string b) {
	if (a.size() == 0 || b.size() == 0)return "0";
	string res = "";
	for (int i = 0; i < b.size(); i++) {
		if (b[i] == '0') {
			//什么都不做
		}
		else {
			for (int j = 0; j < b[i] - '0'; j++) {
				res = stradd(res, a);
			}
		}
		if (i != b.size() - 1)res += "0";
	}
	
	return res;
}
//将一个int转成字符串形式
string trans(int b) {
	string res = "";
	while (b != 0) {
		res .insert( 0,1,(char)((b % 10) + '0'));
		b /= 10;
	}
	return res;
}
//返回n的阶乘
string f(int n) {
	string res = "1";
	for (int i = 1; i <= n; i++) {
		res = strmul(res, trans(i));
	}
	return res;
}

void test() {
	string s1 = "81";
	string s2 = "3";
	cout << strmul(s1, s2) << endl;
	int a = 9978023;
	cout << trans(a) << endl;
}
int main() {
	//test();
	int N;
	while (cin >> N) {
		cout << f(N) << endl;
	}
	return 0;
}