Define a struct type, named ListNode,
with two fields - one is an int and has the field name
key, the other is a pointer to a
ListNode and
has the field name next.
Sample solution
struct ListNode {
int key;
ListNode *next;
};
Write a function named print that takes a pointer
to a ListNode as its only parameter, and
(assuming the parameter is a pointer to the front of a list)
prints the contents of the linked list, displaying each
node's key on a different line.