C :

#include "stdio.h"
int main()
{ 
unsigned int n,b;
int i,j;
char a[1024];
while(scanf("%d%d",&n,&b)!=EOF)
{	
i=0;
while(n)
{
if(b>=10)
{
if(n%b==10) a[i]='A';					
else if(n%b==11) a[i]='B';
else if(n%b==12) a[i]='C';
else if(n%b==13) a[i]='D';
else if(n%b==14) a[i]='E';
else if(n%b==15) a[i]='F';
else a[i]=n%b+'0';
}
else a[i]=n%b+'0';
n/=b;
i++;
}
if(b==16)
printf("0X");
for(j=i-1;j>=0;j--)
printf("%c",a[j]);
printf("\n");
}
return 0;
}

C++ :

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>

char  *str ="0123456789ABCDEF";
void print(int n,int base){
     if(n){
           print(n/base,base);
           printf("%c",str[n%base]);
     }

}
int main(){
    int n,base;
    int count;
  
   
  
    while(scanf("%d %d",&n,&base) == 2){
                    if(base == 16){
                            printf("0X");
                    }
                    if(!n){
                           printf("0\n");
                           continue;
                    }
                    print(n,base);
                    printf("\n");
    }
    
    return 0;
} 

Java :

import java.util.Scanner;

public class Main
{
	public static void main(String[] args)
	{
		Scanner scanner = new Scanner(System.in);
		while(scanner.hasNext())
		{
		long N = scanner.nextLong();
		int B = scanner.nextInt();
		if(N>1000000000||N<0) return;
		if(B>16||B<2) return;
		if (B == 16) System.out.println("0X" + Long.toString(N, B).toUpperCase());
		else
		System.out.println(Long.toString(N, B).toUpperCase());
		}
	}
}