void revert(struct node **root) {
     //jednoprvkovy
     if((*root)->next == NULL)
         return;

     //dvoj a viac prvkovy
     struct node *act = *root;
     struct node *previous = NULL;
     struct node *nxt;
     while(act->next != NULL) {
         nxt = act->next;
         act->next = previous;
         previous = act;
         act = nxt;
     }
     *root = act;

 }