The three-parameter constructor for Doubly Linked Nodes makes use of two if statements. Suppose that you replace the calls to this constructor with the one-parameter constructor and manually use set...

The three-parameter constructor for Doubly Linked Nodes makes use of two if statements. Suppose that you replace the calls to this constructor with the one-parameter constructor and manually use set Next and set Previous to set the appropriate references. The if statements disappear. Why?

Even though we don’t need to have the special cases in, for example, the indexed version of add, it is desirable to handle one or more cases in a special way. What are the cases, and why is it desirable?