int depth(treenode *p)
{
   if(p==NULL)return -1 ;
   if(p->left){h1=depth(p->left);}
   if(p=>right){h2=depth(p->right);}
   return(max(h1,h2)+1);
}



Check the boundary condition for p =NULL means no tree exists.
so depth should be -1 if only root node is there then depth
of the tree is 0.