大数相加-cpp

#include<iostream>
#include<string>
#include<algorithm>
using namespace std;
int main(){
    string a,b,c;
    int length_a,length_b;
    cin>>a;
    cin>>b;
    length_a=a.length();
    length_b=b.length();
    reverse(a.begin(),a.end());
    reverse(b.begin(),b.end());
    if(length_a>=length_b)
    {
        for(int i=0;i<(length_a-length_b);i++)
        {
            b.append(1,'0');
        }
    }
    else
    {
        for(int i=0;i<(length_b-length_a);i++)
        {
            a.append(1,'0');
        }
    }

    
cout<<a<<endl;
cout<<b<<endl;

    if(length_a>=length_b)
    {
        for(int i=0;i<length_a;i++)
        {
            a[i]-='0';
            b[i]-='0';
            if((a[i]+b[i])>=10)
            {
                c.append(1,(a[i]+b[i])%10);
                a[i+1]++;
                c[i]+='0';
            }
            else
            {
                c.append(1,a[i]+b[i]);
                c[i]+='0';
            }

        }
    }
    else
    {
        for(int i=0;i<length_b;i++)
        {
            a[i]-='0';
            b[i]-='0';
            if((a[i]+b[i])>=10)
            {
                c.append(1,(a[i]+b[i])%10);
                b[i+1]++;
                c[i]+='0';

            }
            else
            {
                c.append(1,a[i]+b[i]);
                c[i]+='0';

            }

        }

    }

    
    reverse(c.begin(),c.end());
    int length_c=c.length();

    cout<<c;
}