Operations on linked lists

Suppose that we have already created a linked list in memory, and that a variable lis of type ListNode contains a reference to the first element of the list.

We can perform various operations on such a list. The most common operations are:

• checking whether the list is empty;
• accessing a node to modify it or to obtain the information in it;
• traversing the list to access all elements (e.g., to print them, or to find some specific element);
• determining the size (i.e., the number of elements) of the list;
• inserting or removing a specific element (e.g., the first one, the last one, or one with a certain value);
• creating a list by reading the elements from an input stream;
• converting a list to and from an array, string, etc.

Note that some of the operations above do not modify the list at all, some modify only the information field of a node, and some modify the structure of the list, by changing how the nodes are connected to each other. We will realize each operation through a static method:

• The method takes as one of its parameters a reference to the first node of the list.
• If the method modifies the list, it also returns a reference to the first node of the modified list as its return value.