注意0时的情况 

#include <iostream>
using namespace std;
char ans[100];
char chr[] = {'A','B','C','D','E','F'};
int main(){
	int a,k = 0;
	cin>>a;
	if(a == 0) cout<<"0"<<endl;
	while(a > 0){
		int yu = a % 16;
		a = a / 16;
		if(yu < 10){
			ans[k++] = (char)yu + '0';
		}else{
//			cout<<chr[yu - 10]<<endl;
			ans[k++] = chr[yu - 10];
		}
	}
	for(int j = k - 1; j >= 0; j--){
		cout<<ans[j];
	}
	return 0;
}