C++大數相加

大數不能看成單純的數，得用字符來替代

#include<iostream>
#include<string>
using namespace std;
string account(string num1,string num2)
{
if(num1.size()<num2.size())
{
string temp=num1;
num1=num2;
num2=temp;
}
int length1=num1.size(),length2=num2.size(),flag=0,a,b,sum;
while(length1>0)
{
a=num1[length1-1]-'0';
if(length2>0)
b=num2[length2-1]-'0';
else
b=0;
sum=a+b+flag;
if(sum>=10)
{
num1[length1-1]='0'+sum%10;
flag=1;
}
else
{
num1[length1-1]='0'+sum;
flag=0;
}
length1--;
length2--;
}
if(1==flag)
num1="1"+num1;
return num1;
}
int main()
{
int t;
int x=1,n,m;
cin>>t;
while(t--)
{
string a,b;
cin>>a>>b;
cout<<"Case "<<x<<":"<<endl;
cout<<a<<" + "<<b<<" = "<<account(a,b)<<endl;
if(t!=0)
cout<<endl;
x++;
}
return 0;
}