[c++] 大整数乘法（字符串乘法）

#include<iostream>
#include<algorithm>
#include<string>
using namespace std;

string add(string a,string b)
{
	if(a.length()==0)
		return b;
	if(b.length()==0)
		return a;
	a.length()<b.length()?a.swap(b):a.length();
	
	reverse(a.begin(),a.end());
	reverse(b.begin(),b.end());
	int flag=0;
	for(int i=0;i<a.length();i++)
	{
		int aInt=a[i]-'0';
		int bInt=0;
		if(i<b.length())
			bInt=b[i]-'0';
		int result=aInt+bInt+flag;
		a[i]=result%10+'0';
		flag=result/10;
	}
	if(flag!=0)
	{
		a=a+(char)(flag+'0');
	}
	reverse(a.begin(),a.end());
	return a;
}
string multiply(std::string strMultiplierA,char x)
{
	int b=x-'0';
	int flag=0;
	for(int i=strMultiplierA.length()-1;i>=0;i--)
	{
		int a=strMultiplierA[i]-'0';
		int result=a*b+flag;
		strMultiplierA[i]=result%10+'0';
		flag=result/10;
	}
	if(flag!=0)
		strMultiplierA=(char)(flag+'0')+strMultiplierA;
	while(strMultiplierA.length()>1&&strMultiplierA[0]=='0')
		strMultiplierA=strMultiplierA.substr(1,strMultiplierA.length());
	return strMultiplierA;
}

/*****************************************************************************
 Prototype    : multiply
 Description  : 两个任意长度的长整数相乘, 输出结果
 Input Param  : 
                const std::string strMultiplierA  乘数A
                const std::string strMultiplierB  乘数B
 Output       : 
                std::string strRst            乘法结果
 Return Value : 
                int                       0  正确  
                                         -1  异常
*****************************************************************************/
int multiply (const std::string strMultiplierA,const std::string strMultiplierB, std::string &strRst) 
{

    /* 在这里实现功能 */
    if(strMultiplierA.length()<=0||strMultiplierB.length()<=0)
		return -1;
	bool flag=false;//false"+"  true"-"
	string strA=strMultiplierA,strB=strMultiplierB;
	if(strMultiplierA[0]=='-')
	{
		flag=~flag;
		strA=strMultiplierA.substr(1,strMultiplierA.length());
	}
	if(strMultiplierB[0]=='-')
	{
		flag==true?flag=false:flag=true;
		strB=strMultiplierB.substr(1,strMultiplierB.length());
	}

	for(int i=strB.length()-1;i>=0;i--)
	{
		string result=multiply(strA,strB[i]);
		int j=i;
		while(++j<strB.length())
			result=result+"0";
	//	while(result.length()>1&&result[0]=='0')
	//		result=result.substr(1,result.length());
		strRst=add(strRst,result);
	}
	while(strRst.length()>1&&strRst[0]=='0')
		strRst=strRst.substr(1,strRst.length());
	if(flag==true&&strRst!="0")
		strRst='-'+strRst;
    return 0;
}

int main()
{
	std::string strResult = "";

	 multiply("-5489324", "0", strResult);

	 cout<<strResult<<endl;
}