int depth(treenode *p)
{
   if(p==NULL)return(0);
   if(p->left){h1=depth(p->left);}
   if(p=>right){h2=depth(p->right);}
   return(max(h1,h2)+1);
}

The above code will return the number of nodes in the
longest path from root to the leaf. So subtracting with -1
will give the depth of the tree. Depth of the tree is the
distance from root to the deepest leaf.