Scaderea a 2 nr mari

#include <fstream>
#include <cstring>

using namespace std;

ifstream fin("scad.in");
ofstream fout("scad.out");

int na, nb;
char a[1005], b[1005];

void compare(){
    /// daca a < b inversez vectorul a cu vectorul b
    if(nb > na){
        for(int i = 0; i < nb; i++)
            swap(a[i], b[i]);

        // pot folosi si swap(na, nb)
        int c = nb;
        nb = na;
        na = c;
        return;
    }

    if(nb == na){
        int ok = true;  /// it is true that na > nb
        for(int i = 0; i < nb; i++){
            if(b[i] > a[i]){
                ok = false;
                break;
            }
        }
        if(ok == false){
            for(int i = 0; i < nb; i++)
                swap(a[i], b[i]);
            // pot folosi si swap(na, nb)
            int c = nb;
            nb = na;
            na = nb;
        }
    }
}


void scad(){
    int t = 0;
    for(int i = 0; i < na; i++){
        if(a[i] >= b[i])
            a[i] = ((a[i] - '0') - (b[i] - '0')) + '0';
        else{
            int j = i + 1;
            while(a[j] == '0')
                a[j++] = '9';
            a[j]--;
            a[i] = (10 + (a[i] - '0') - (b[i] - '0')) + '0';
        }
    }
    /// verific daca mai am 0-uri nesemnificative dupa scadere
    for (; na > 0 && a[na - 1] == '0'; na--); // sa n-am zerouri nesemnificative

}

int main()
{
    fin.getline(a, 1001);
    fin.getline(b, 1001);

    na = strlen(a);
    nb = strlen(b);

    compare();    /// daca a < b inversez cifrele din a cu cifrele din b

    /// retin numarul pe invers
    for(int i = 0; i < na / 2; i++){
        swap(a[i], a[na - i - 1]);
        swap(b[i], b[nb - i - 1]);
    }

    /// egalez nr de cifre
    if(na < nb){
        for(int i = na; i < nb; i++)   a[i] = '0';
        na = nb;
    }
    else if (nb < na){
        for(int i = nb; i < na; i++)   b[i] = '0';
        nb = na;
    }

    /// fac functia de scadere, stiind ca a >= b, lucru verificat in comparare()
    scad();

    for(int i = na - 1; i >= 0; i--)
        fout << a[i];
    fout << '\n';
    return 0;
}