用栈就好啦~~

注意，16进制需要特殊转换，所以开数组；

#include<bits/stdc++.h>
using namespace std;
stack<int>s;
char a[16]={'0','1','2','3','4','5','6','7','8','9','A','B','C','D','E','F'};
long long n,t;
int main(){
	cin>>n>>t;
	while(n){
		s.push(n%t);
		n/=t;
	}
	while(!s.empty()){
		cout<<a[s.top()];
		s.pop();
	}
	cout<<endl;
	return 0;
}