C :

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

char num[]="0123456789ABCDEF";
int ascii[128]={0};

void Init()
{
  for(int i=0;i<=9;i++)
    ascii[i+'0']=i;
  for(int i=0;i<=5;i++)
  {
    ascii['a'+i]=10+i;
    ascii['A'+i]=10+i;
  }
}

long Strnum(char str[],int hex)
{
  long ans=0;
  int i=0;
  while(str[i])
  {
    ans=ans*hex+ascii[str[i]];
    i++;
  }
  return(ans);
}

char *Numstr(char str[],long number,int hex)
{
  str[0]='0';
  int i=0;
  while(number)
  {
    str[i]=num[number%hex];
    number/=hex;
    i++;
  }
  if(i)
    str[i]='\0';
  else
    str[1]='\0';
  int len=strlen(str);
  char chartmp;
  for(i=0;i<len/2;i++)
  {
     chartmp=str[i];
     str[i]=str[len-i-1];
     str[len-i-1]=chartmp;
  }
  return(str);
}
main()
{
	Init();
    int a,b;
	char str[100];
	while(scanf("%d%s%d",&a,str,&b)!=-1)
	{
		long number=Strnum(str,a);
		puts(Numstr(str,number,b));
	}
}

C++ :

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

char result[1000];

void base_convert(char num[],int s,int e)
{
	int i,flag,k,j,a[1000],b[1000];
	for(i=0;i<strlen(num);i++)
	{
		if(num[i]>='a'&&num[i]<='f')
			a[i]=num[i]-'a'+10;
		else if(num[i]>='A'&&num[i]<='F')
			a[i]=num[i]-'A'+10;
		else
			a[i]=num[i]-'0';
	}
	k=0;
	while(1)
	{
		for(i=0;i<strlen(num);i++)
		{
			if(a[i]!=0&&i<strlen(num)-1)
			{
				a[i+1]+=(a[i]%e)*s;
				a[i]/=e;
			}
			if(i==strlen(num)-1)
			{
				b[k++]=a[i]%e;
				a[i]/=e;
			}
		}
		flag=0;
		for(i=0;i<strlen(num);i++)
			if(a[i]!=0)
				flag=1;
		if(!flag)
			break;
	}
	for(j=0,i=k-1;i>=0;i--)
	{
		if(b[i]>=10&&b[i]<=15)
			result[j++]=b[i]+'A'-10;
		else
			result[j++]=b[i]+48;
	}
	result[j]='\0';
}

int main()
{
	int a,b;
	char n[1000];
	while(scanf("%d%s%d",&a,n,&b)!=EOF)
	{
		base_convert(n,a,b);
		puts(result);
	}
	return 0;
}

Pascal :

var mst,st,st1:string;
    s,i,j,len,m,n,o,p:longint;k:qword;
    a,b:array[1..10000] of integer;

begin
   while not eof do
     begin
       readln(mst);
       p:=pos(' ',mst); val(copy(mst,1,p-1),m);delete(mst,1,p);
       p:=pos(' ',mst); st:=copy(mst,1,p-1); st:=upcase(st);delete(mst,1,p);
       val(mst,n);

       len:=length(st);
       for i:=1 to len do
          if st[i] in ['0'..'9']  then a[len+1-i]:=ord(st[i])-48
                 else  a[len+1-i]:=ord(st[i])-55;

        k:=0;s:=1;
       for i:=1 to len do
         begin
            k:=k+a[i]*s;
            s:=s*m;
          end;

       i:=0;
       repeat
          inc(i);
          b[i]:=k mod n;
          k:=k div n;
        until k=0;

     for j:=i downto 1 do
        if b[j]<10 then write(chr(b[j]+48)) else write(chr(b[j]+55));
     writeln;
     end;
end.

Java :

import java.util.*;
import java.math.*;

public class Main {

public static void main(String args[]){

Scanner cin=new Scanner(System.in);

while(cin.hasNext()){

int a=cin.nextInt();

BigInteger c=new BigInteger(cin.next(),a);

int b=cin.nextInt();

System.out.println(c.toString(b).toUpperCase());

}

}
}