进制转换

#include <iostream>
using namespace std;
const int t=20000000;
char a[t],b[t];
int fun(char c)
{
    int k;
    if (c>='1'&&c<='9')
    {
        k=c-'0';
        return k;
    }
        k=c-55;
        return k;
}
int fun1(char x)
{
    int k;
    if (x>=0&&x<=9)
    {
        k=x+'0';
        return k;
    }
    k=x+55;
    return k;
}
int main()
{
    int n,m,i,j,sum=0;
    cin >> n;
    cin.get();
    cin.getline(a,t);
    cin >> m;
    for (i=0;a[i]!='\0';i++)
    {
        if (a[i]=='0')
        {
            sum*=n;
        } 
        else
        {
          sum=sum*n+fun(a[i]);
        }
    }
    j=0;
    while (sum!=0)
    {
        b[j]=fun1(sum%m);
        sum=sum/m;
        j++;
    }
    for (i=j-1;i>=0;i--)
    {
        cout << b[i];
    }
    cout << endl;
    return 0;
}