Deleting the first occurrence of an element from a list

To delete the first occurrence of an element elem from a list through iteration:

• we scan the list searching for elem;
• if elem does not appear in the list, then we do nothing;
• otherwise, depending on where elem appears in the list, we distinguish three cases:
  1. elem is the first element of the list: we return the reference to the second element of the list;
  2. elem is neither the first element nor the last element of the list: we update the next field of the element preceding elem in such a way that it points to the element following elem;
  3. elem is the last element of the list: as case 2, with the only difference that the next field of the preceding element is set to null.

Observations:

• To be able to update the next field of the element preceding elem, we have to stop scanning the list when we have a reference to the element preceding elem (and not when we have reached elem itself).

  Example: Suppose we have to delete the element "B" from the list shown in the figure:

  

• To stop scanning the list after we have found and deleted the element, we use a boolean sentinel.
• To avoid having to deal separately with the first element, we can again use the generator node technique.

Iterative implementation:

public static ListNode delete(ListNode lis, String s) {
  ListNode p = new ListNode();    // create the generator node
  p.next = lis;
  lis = p;

  boolean found = false;
  while ((p.next != null) && !found) {
    if (p.next.info.equals(s)) {
      p.next = p.next.next;       // delete the element
      found = true;               // forces exit of the loop
    } else
      p = p.next;
  }

  return lis.next;                // delete generator node
}

The recursive characterization and implementation are left as an exercise.