Hint 1 (size method)

Small hint: 

We can do a postorder traversal of the tree. This will allow us to count the number of nodes in the tree from the bottom up. For example, the size of a leaf nodes children (empty trees) are 0, the size of the subtree rooted at a leaf node is 1, etc. What is the size of the subtree rooted at that leaf node's parent?

Since this is a recursive method, we will need a base case and a recursive part.
    What is the base case? (When a subtree is empty. But how do we check for this?)
    What is the recursive part? (When the tree is not empty)







Big hint:

Algorithm
    If the tree is empty, return a size of 0
    If the tree is not empty
        Get the size of the left subtree (recursive call)
        Get the size of the right subtree (recursive call)
        Return the size of myself + my left subtree + my right subtree