Implement a singly linked list with a header node, along with a pointer to
the header node, but no tail node as a class template named singleList.
Include methods to
a. return the size of the linked list,
b. print the linked list,
c. test if a value x is contained in the linked list,
d. add a value x if it is not already contained in the list,
e. remove a value x if it is contained in the linked list.
You need not include iterators or functions begin and end. Include a main
function that instantiates a list of integers, singleList, and allows an
interactive user to execute any of the five member functions listed above.