Linked Lists

• What is a linked list?
  • An ordered collection of elements where each element, except for the first has an immediate predicessor, and each element, except for the last has an immediate successor.
  • An empty list is possible and is a list containing no elements.
  • Often charactarized by constant time insertion and deletion.
  • Operations include
    • First, last, next, previous
    • insert, delete
    • find
  • Performance characteristics:
    • We expect everything but find to be O(1)
    • We expect find to be linear.
• Implementation
  • Normally associated with a link and a data field (singly linked lists)
  • Or a data field, a forward link and a backward link. (Doubly linked lists)
  • Normally a pointer to the first (head) and last (tail) of the list are maintained.
  • In some implementations, an empty node represents a singly linked list.
  • In some doubly linked applications, an empty head and tail node are kept
• Usage
  • Lists are normally employed when there is a need to maintain order and
  • There will be many insertions and deletions.
  • And there is not a major need for random access.
  • Consider, for example if we wish to delete every other item in the structure. O(n²) in an array, O(n) in a list.
• The STL list class.
  • #include <list>
  • This implements a doubly linked list.
  • list<type> identifier
  • Operations:
    • push_back, push_front
    • pop_front, pop_back
    • bool empty, int size
    • clear removes all items
    • remove(item), removes all instances of the item
    • front, back returns first or last item in the list
    • sort sorts the list.
    • = is supported
  • Iterators:
    • list<type>::iterator id
    • list<type>::reverse_iterator id
    • begin() returns a forward iterator to the first item
    • end() returns a forward iterator to one past the last item

    • list<int> L;
      list<int>:: iterator i;
          for(i=L.begin();i!=L.end();i++) {
      }
      	       

    • rbegin and rend for reverse iterators
    • insert(iterator, value), inserts before the iterator
    • erase(iterator)
    • erase(iterator, iterator)
    • when an item is erased from the list, any iterators pointing to this item are no longer valid
• Some code