C++ :

#include <cstdio>
int main(){
    long long n;
    if(scanf("%lld",&n)!=1) return 0;
    if(n==0){ printf("0\n"); return 0; }
    const char*H="0123456789ABCDEF";
    char b[32]; int k=0;
    while(n>0){ b[k++]=H[n&15]; n>>=4; }
    for(int i=k-1;i>=0;i--) putchar(b[i]);
    putchar('\n');
    return 0;
}