Biegral Blog

#include <iostream>
using namespace std;

int multi(char a, char b)
{
	return (a - '0') * (b - '0');
}

void strMulti(char *a, char *b, char *c)
{
	// 最好对a,b和c的合法性进行判断

	int lenA = strlen(a);
	int lenB = strlen(b);
	int maxLen = lenA + lenB;
	int *p = new int[maxLen];
	memset(p, 0, maxLen * sizeof(int)); // 千万要注意不要写成memset(p, 0, maxLen);

	int i, j;
	for(j = lenB - 1; j >= 0; j--)
	{
		for(i = lenA - 1; i >= 0; i--)
		{
			p[j + i + 1] += multi(b[j], a[i]);
		}
	}

	// 处理进位操作
	for(i = maxLen - 1; i >= 1; i--)
	{
		p[i - 1] += p[i] / 10;
		p[i] = p[i] % 10;
	}

	int *s = p;

	// m位正整数和n位正整数相乘，结果的位数必然是(m+n-1)位或者(m+n)位
	if(0 == p[0])
	{
		p++;
	}
	
	int *pTmp = NULL;
	for(pTmp = p; pTmp < s + maxLen; pTmp++)
	{
		*c++ = *pTmp + '0';
	}

	*c = '\0';

	delete s;
}

int main()
{
	char a[101] = "123456789";
	char b[101] = "987654321";
	char c[201] = "";
	strMulti(a, b, c);
	cout << c << endl; // 121932631112635269
	
	return 0;
}