Check if 2 Binary Trees are isomorphic

Two Binary Trees are known as isomorphic if one of them can be obtained from the other one by series of flipping of nodes, swapping the children both left and right of number of nodes. Any number of nodes at all levels can swap their child nodes.

With above definition we can say that two empty trees are isomorphic.
Let us consider an example:

Above trees are isomporhpic.
We can swap node 2 and 3. Then we can swap node NULL and 6. Then finally we can swap node 7 and 8.
We have two main approache...