unordered_set(Hashmap) to store trees

I was thinking to store trees in an unordered set in order to pick the unique trees from a collection of trees. If my tree node has representation like described below, how to come up with the hash function for that ?

struct node {
      int val;
      node *left;
      node *right; 
};

Edit: By similar tree, I mean that they have the same values in all the respective nodes. So, compare function will be using the actual trees to compare rather than memory address of root.