大数之乘法加法除法

加法（不成熟的小代码orz）

#include <iostream>
#include <string>
#include <cmath>
using namespace std;
int main()
{
	string a, b;
	int res[500] = {0};
	cin >> a >> b;
	if(a.length() > b.length())
	{
		string strb(a.length()-b.length(), '0');
		b = strb + b;
	}
	else
	{
		string stra(b.length()-a.length(), '0');
		a = stra + a;
	}
//	cout << a << " " << b << endl;
	for(int i = max(b.length(), a.length()) - 1; i > 0; i--)
	{
		res[i] += ((a[i]-'0')+(b[i]-'0'))%10;
//		cout << ((a[i]-'0')+(b[i]-'0'))%10 << " ";
//		cout << res[i] << endl;
		res[i-1] = ((a[i]-'0')+(b[i]-'0'))/10;
	}
	res[0] += ((a[0]-'0')+(b[0]-'0'))%10;
//	cout << res[0] << endl;
	if(((a[0]-'0')+(b[0]-'0'))/10 > 0)
		cout << ((a[0]-'0')+(b[0]-'0'))/10;
	for(int i = 0; i < max(b.length(), a.length()); i++)
		cout << res[i];
}

乘法来源（蓝桥杯 矩阵翻硬币）：点击打开链接

另外学到一点：一个数有奇数个约数 -> 完全平方数，因为每个数为两个（即一对）数相乘得到，完全平方数的两个数为同一个，所以为奇数个。

#include <iostream>
using namespace std;
int main()
{
	string a, b, num = "";
	int res[10010] = {0};
	int i;
	cin >> a >> b;
	int lena = a.length();
	int lenb = b.length();
	for(i = 0; i < lena; i++)
	{
		for(int j = 0; j < lenb; j++)
		{
			res[lena-1+lenb-1-i-j] += (a[i]-'0')*(b[j]-'0');
		}
	}
	for(i = 0; i < lena+lenb; i++)
	{
		res[i+1] += res[i]/10;
		res[i] %= 10;
	}
	for(i = lena+lenb; i >= 0; i--)
	{
		if(res[i]!=0)
			break;
	}
	for( ; i >= 0; i--)
	{
		num += (char)(res[i]+'0');
	}
	cout << num << endl;
}

除法

#include <iostream>
using namespace std;
int main()
{
	string a;
	int b, flag = 0;
	cin >> a >> b;
	int temp = 0;
	for(int i = 0; i < a.length(); i++)
	{
		temp = a[i]-'0' + temp*10;
//		cout << temp << " ";
		if(temp >= b)
		{
			cout << temp/b;
			flag = 1;
		}
		else if(flag)
		{
			cout << 0;
		}
		temp %= b;
//		cout << temp <<  endl;
	}
	if(!flag)
	{
		cout << 0 << endl;
	}
	else
	{
		cout << " " << temp;
	}
}