题解 | #N的阶乘#

//土尔逊Torson 编写于2023/05/22
#define _CRT_SECURE_NO_WARNINGS
#include <iostream>
#include <cstdio>
#include <string>
#include <cstring>

using namespace std;

const int MAXN = 10000;

struct BigInteger083 { //定义大数
	int digit[MAXN];
	int length;
	BigInteger083();
	BigInteger083(int x);
	BigInteger083 operator=(string str);
	BigInteger083 operator*(const BigInteger083& b);
	friend istream& operator>>(istream& in, BigInteger083& x);
	friend ostream& operator<<(ostream& out, const BigInteger083& x);
};

istream& operator >> (istream& in, BigInteger083& x) { //定义输入操作符
	string str;
	in >> str;
	x = str; //使用本程序定义的 赋值操作符 （ = ）
	return in;
}

ostream& operator << (ostream& out, const BigInteger083& x) { //定义输出操作符
	for (int i = x.length - 1; i >= 0; --i) {
		out << x.digit[i];
	}
	return out;
}

BigInteger083::BigInteger083() { //定义大数重载
	memset(digit, 0, sizeof(digit));
	length = 0;
}

BigInteger083::BigInteger083(int x) { //定义大数 int 重载
	memset(digit, 0, sizeof(digit));
	length = 0;
	if (x == 0) {
		digit[length++] = x;
	}
	while (x != 0) {
		digit[length++] = x % 10;
		x /= 10;
	}
}

BigInteger083 BigInteger083::operator=(string str) { //定义赋值操作符 （ = ）
	memset(digit, 0, sizeof(digit));
	length = str.size();
	for (int i = 0; i < length; ++i) {
		digit[i] = str[length - i - 1] - '0';
	}
	return *this;
}

BigInteger083 BigInteger083::operator*(const BigInteger083& b) { //定义乘法操作符 ( * )
	BigInteger083 answer;
	answer.length = length + b.length;
	for (int i = 0; i < length; ++i) { //用传统纸上手算乘法的方式计算，暂时不进位
		for (int j = 0; j < b.length; ++j) {
			answer.digit[i + j] += digit[i] * b.digit[j];
		}
	}
	for (int i = 0; i < answer.length; ++i) { //处理进位，进行进位运算
		answer.digit[i + 1] += answer.digit[i] / 10;
		answer.digit[i] %= 10;
	}

	//注意 answer.digit[answer.length - 1] 等于 0 时 就是第 answer.digit[] 的 第 answer.length 个数 等于 0
	while (answer.digit[answer.length - 1] == 0 && answer.length > 1) { //因为是逆置的大数，所以从后向前去掉多余的 0
		answer.length--;                                            //和重置长度参数
	}
	return answer;
}

int main() {
	int n;
	while (scanf("%d", &n) != EOF) {
		BigInteger083 Q(1);
		for (int i = 1; i <= n; ++i) { //计算N的阶乘，从小数往大数乘法计算
			Q = Q * BigInteger083(i);
		}
		cout << Q << endl; //用定义的输出操作符输出
	}
	system("pause");
	return EXIT_SUCCESS;
}
// 64 位输出请用 printf("%lld")