searching value in binary_tree

i have problem with these functions that return the id of a node of a binary tree.

int binary_tree::get_id_from_key(int key)
node *aux=search(root,key);
return aux->id_node;

node* binary_tree::search(node *&ptr,const int n)

return ptr;
else if(n>ptr->key)
return search(ptr->right,n);
else return search(ptr->left,n);
else return 0;

the question is: is it right to return 0 if i don' t find the node containing value n?
in this case what should i return in the function get_id_from_key?

thanks in advance guys
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