LeetCode Multiply Strings

面试那时候听说过大整数乘法，当时加法都够呛，要真碰到乘法肯定吓尿了，这里碰到了，想了想，写了写，也不过如此。

// LeetCode_MultiplyStrings.cpp : 定义控制台应用程序的入口点。
//

#include "stdafx.h"
#include <iostream>
#include <string>
#include <algorithm>
using namespace std;

string add(const string &str1,const string &str2)
{
	string ret="";
	int lenstr1 = str1.length();
	int lenstr2 = str2.length();
	if (lenstr1==0)
		return str2;
	if(lenstr2==0)
		return str1;
	bool flag = false;
	int i = lenstr1-1, j = lenstr2-1;
	while(i>=0&&j>=0)
	{
		int tempbit = str1[i] - '0' + str2[j] - '0';
		if(flag)
			tempbit++;
		if (tempbit>=10)
		{
			tempbit = tempbit - 10;
			flag = true;
		}
		else
			flag = false;
		char tempchar = tempbit + '0';
		ret += tempchar;
		--i;--j;
	}
	while(i>=0)
	{
		int tempbit = str1[i] - '0';
		if(flag)
			tempbit++;
		if (tempbit>=10)
		{
			tempbit = tempbit - 10;
			flag = true;
		}
		else
			flag = false;
		char tempchar = tempbit + '0';
		ret += tempchar;
		--i;
	}
	while(j>=0)
	{
		int tempbit = str2[j] - '0';
		if(flag)
			tempbit++;
		if (tempbit>=10)
		{
			tempbit = tempbit - 10;
			flag = true;
		}
		else
			flag = false;
		char tempchar = tempbit + '0';
		ret += tempchar;
		--j;
	}
	if(flag)
		ret += "1";
	reverse(ret.begin(),ret.end());
	return ret;
}
string multiply(string num1, string num2) {
	if(num1=="0"||num2=="0")
		return string("0");
	if(num1 == "") return num2;
	if(num2 == "") return num1;
	int lennum1 = num1.length();
	int lennum2 = num2.length();
	int i = lennum1 - 1,j = lennum2 - 1;
	string ret = "";
	for (i=lennum1-1;i>=0;i--)
	{
		int tempbiti = num1[i] - '0';
		string onesum = "";
		int flag = 0;//进位
		for (j=lennum2-1;j>=0;j--)
		{
			int tempbit = tempbiti * (num2[j] - '0');
			tempbit += flag;
			flag = tempbit/10;
			tempbit = tempbit%10;
			char tempchar = tempbit + '0';
			onesum += tempchar;
		}
		if (flag)
		{
			onesum += (flag + '0');
		}
		reverse(onesum.begin(),onesum.end());
		for (int k=lennum1-1;k>i;k--)
		{
			onesum += "0";
		}
		ret = add(onesum,ret);
	}
	return ret;	
}

int _tmain(int argc, _TCHAR* argv[])
{
	string str1("129"),str2("889");
	while(cin>>str1&&cin>>str2)
	cout<<multiply(str1,str2)<<endl;
	//cout<<add(str1,str2)<<endl;
	system("pause");
	return 0;
}