Hint 1

• If the current node r is null, return 0.
• Define an integer variable to count the number of nodes. Initialize this variable to 1 to count the current node r.
• If the left child of the current node is not null, then invoke the algorithm recursively with parameter equal to the left child of the current node; add the value returned by the recursive call to your counter.
• Do the same thing with the right child of the current node.
• Return the value of the counter.