用短除法。

#include<bits/stdc++.h>
using namespace std;
string card="0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";
string ans="";
int main(){
    int n,m;
    cin>>n>>m;
    if(n==0){
        cout<<0;
        return 0;
    }
    while(n>=1){
        int tmp=n%m;
        ans+=card[tmp];
        n/=m;
    }
    reverse(ans.begin(),ans.end());
    cout<<ans;
}