3. Find a recursive definition for Preorder: binary Trees (T) lists (T) Where Preorder (T) is the list of nodes from an preorder traversal of T. (4 points). 4. Count the number of strings of length 4 over A = {a, b, c, d, e, f} that begins with either a or b and have exactly one d. (8 points). 5. Use the pigeonhole principle to determine how many people are needed in a group to say that ten were born in the same month. (4 points)