void nivel(ABin a, int k, SList *l, int level){
    if (!a) return;
    if(k == level){
     SList n = (SList)malloc(sizeof(struct slist));
     n->value = a->value;
     n->next=(*l);
     (*l) = n;
     return;
    }else{
        nivel(a->left, k, l, level+1);
        nivel(a->right, k, l, level+1);
    }
}

typedef struct slist
{
    int value;
    struct slist* next;
} *SList;

typedef struct arvbin* ABin;
typedef struct arvbin
{
    int value;
    ABin right;
    ABin left;
} arvb;