An introduction to Linked Lists.

• code for today
• We will finish the semester by looking at linked lists.
• Note the word linked.
• This will allow us to form a structure where inserts and deletes are extremely fast ($O(1)$)
• The big idea
  • Each piece of data will be stored in a node.
  • Like a train car, this will hold
    • Some data
    • A link to the next node.
  • We will implement this with a struct

    • struct NodeT {
           int data;
           NodeT * next;
      } 

    • 
    • This is a recursive definition (sort of).
    • A NodeT is defined in terms of a NodeT *.
    • We can get away with this because all pointers are of the same size.
  • What's more, we don't want to put the definition of NodeT in the header file.
  • So we will use a forward declaration
    • This is like a function prototype.

    • ...
      #define LIST_T
      
      struct NodeT;
      
      class ListT{
      ...  

• Nothing else in the public part of the list declaration changes.
• But the private data and methods are different.
  • Find now returns a NodeT *
  • I changed the type of current to be a NodeT *
  • I have eliminated data.
  • I have added a variable called head of type NodeT.

  •    private:
           NodeT * Find(int key) const;
           size_t size;
           NodeT * head;
           NodeT * current;
    }; 

• In a linked list, it is customary to have a head pointer.
  • This points at the head of the list.
  • If the list is empty it is nullptr.
  • Otherwise it points to the first node in the list.
  • 
• There are several things you do to prevent errors.
  • No matter what, a pointer must be assigned to a valid piece of memory or nullptr.
  • Always check to make sure the pointer is not nullptr before dereferreencing the pointer.
• The implementation of the linked list has changed quite a bit.
• Let's start by looking at the search function
• 
  • This is a classic pattern for traversing the list.
• Look at the insert function.
• Look at the delete function
• The constructor has changed a little.
• The destructor needs to delete the list, Look at DeleteList.