Two binary search trees T$1$ and T$2$ are said to be isomorphic if they have

the same structure and store the same keys, that is:

$$ both T$1$ and T$2$ consist of a single $($external$)$ node; or

$$ the roots of T$1$ and T$2$ are internal nodes storing items with equal keys, the left

subtree of T$1$ is isomorphic to the left subtree of T$2,$ and the right subtree of T$1$ is

isomorphic to the right subtree of T$2.$