大数运算

#include <iostream>
#include <string>
#include <cassert>
using namespace std;
int main()
{
    string str1, str2, str;
    int length1=0, length2=0,carryBit=0;
	cin >> str1 >> str2;
    length1 = str1.length();
    length2 = str2.length();		
    while(length1>0 || length2>0 )
    {
       int x1=0, x2=0;
       if(length1>0)
       {//str1
	       length1--;
	       x1 = str1.at(length1)-'0';
	       assert(x1>=0 && x1<=9);  //若字符串包含非0~9数字时出错
       }
       if(length2>0)   
       {//str2
	       length2--;
	       x2 = str2.at(length2)-'0';
	       assert(x2>=0 && x2<=9);
        }
	int sum = x1+x2+carryBit;
	char c = sum%10+48;  //把数字转化为字符
   	str += c;
       carryBit = sum/10; //计算进位  
    }//end while
   if(carryBit)
    {
	char c = carryBit+48;
	str += c;
    }
	int i=str.length(), j=i;
	while(j>0)
	{//定位j为非0数字开头
		if('0'==str[j-1])
			--j;
		else
			break;
	}
	if(j!=i)//舍弃0开头，使输出结果以非0开头
	    str.erase(j, i-j);

	for(j=j-1; j>=0; j--)//逆序输出str
		cout << str[j];
	system("pause");
	return 0;
}
//上面str+=c可改为str.insert(1,1,c)则可得到正序结果str；

呃，不知还有没有错