分析：

大數能大到整形類型存儲不了。須要借助於其它的算法，來完畢乘法運算。

能夠使用口算乘法的步驟來模擬乘法操作。例如以下：

123

* 122

=

3 4 6

+ 3 4 6

+ 1 2 3

= 1 5 0 0 6

實現例如以下：

#include<iostream>
#include<string.h>

using namespace std;

char* MutiBigerNum(char* a, char* b)
{
	int len1 = strlen(a);
	int len2 = strlen(b);
	char* res = new char[len1 + len2 + 1];

	memset(res, ‘0‘, len1+len2);
	for(int i=len1-1; i >= 0; i-- )
	{
		int carry = 0;
		for(int j = len2 -1; j >= 0; j--)
		{
			int temp = (res[i + j+ 1] - ‘0‘) + carry + (a[i] - ‘0‘)*(b[j] - ‘0‘);
			carry = 0;
			if(temp >= 10)
			{
				carry = temp/10;
			}
			res[i+j+1] = temp%10 + ‘0‘;
		}
		res[i] = carry + ‘0‘;
	}
	return res;
}

int main(int argn, char** argv)
{
	char *a = "122";
	char *b = "123";
	if(argn > 2)
	{
		a = argv[1];
		b = argv[2];
	}
	char* resnum = MutiBigerNum(a, b);
	if(strlen(resnum) > 0 && resnum[0] == ‘0‘)
		cout << a << "*" << b << "=" << resnum + 1 << endl;
	else
		cout << a << "*" << b << "=" << resnum << endl;
	return 0;
}

./test

122*123=15006

./test 999 888

999*888=887112

49. 搜狗面試題： 大數相乘算法