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;
}

#include<bits/stdc++.h>
using namespace std;
char ch[6]={'A','B','C','D','E','F'};
int jz(int n,int m)
{
	int x[1000],i,d=0;
	while(n!=0)
	{
		d++;
		i=n%m;
		n/=m;
		x[d]=i;
	}
	for(int j=d;j>=1;j--)
	{
		if(x[j]<10)
		{
			cout<<x[j];
	    }
	    else
	    {
	    	cout<<ch[x[j]-10];
		}
	}
}
int main()
{
	int n,m;
	cin>>n>>m;
	jz(n,m);
	return 0;
}