C :

#include<stdio.h>

int main()
{
	int x,n,k,max;
	int ch[500];
	while(scanf("%d%d",&x,&n)==2){
		if(x==0)printf("0");
		max=0;
		k=x>=0?0:1;
		x=x>0?x:-x;
		while(x!=0){
			ch[max++]=x%n;
			x/=n;
		}
		if(k)printf("-");
		while(max>0){
			max--;
			if(ch[max]<10)
				printf("%d",ch[max]);
			else
				switch(ch[max]){
					case 10:printf("A");break;
					case 11:printf("B");break;
					case 12:printf("C");break;
					case 13:printf("D");break;
					case 14:printf("E");break;
					case 15:printf("F");break;
					case 16:printf("G");break;
			}
		}
		printf("\n");

	}
	return 0;
}

C++ :

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <queue>
#include <stack>
#include <map>
#include <algorithm>
using namespace std;
#define max_n 40000
int main()
{
	int a[1000];
	int n,m;
	while(scanf("%d%d",&n,&m)!=EOF)
	{
		int count=0;
		if(n<0)
		{
			printf("-");
		    n=-n;
		}
		if(n==0)
			a[count++]=n;
		while(n)
		{
			a[count++]=n%m;
			n/=m;
		}
		for(int i=count-1;i>=0;i--)
		{
			if(a[i]<10)
				printf("%d",a[i]);
			else
			{
				printf("%c",'A'+a[i]-10);
			}
		}
		printf("\n");
	}
	return 0;
}