3. Find a recursive definition for Preorder: binary Trees(T) list(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)