C :

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#include<ctype.h>
#include<math.h>
char s[10001],f[1];
int main(){
	long long n,i;
	scanf("%lld",&n);
	if(n==0){
		printf("%d",0);
		return 0;
	}
	while(n!=0){
		if(n%16>=10){
			f[0] = n%16-10+'A';
		}else{
			f[0] = n%16+'0';
		}
		strcat(s,f); 
		n = n/16;
	}
	for(i=strlen(s)-1;i>=0;i--){
		printf("%c",s[i]);
	}
	return 0;
}

C++ :

#include <bits/stdc++.h>
using namespace std;
int main()
{
	long long n,a[20],k=0;
	cin>>n;
	while (n!=0)
	{
		a[k]=n%16;
		k++;
		n=n/16;
	}
	for (int i=k-1;i>=0;i--)
	{
		if (a[i]<10) cout<<a[i];
		else cout<<char(a[i]+55);
	}
	return 0;
}

Java :

import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		long number = sc.nextLong();
		System.out.println(Long.toHexString(number).toUpperCase());
		sc.close();
	}

}

Python :

a=int(input())
print(hex(a)[2:].upper())