Doubly Linked List

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In computer science, a doubly linked list is a linked data structure that consists of a set of sequentially linked records called nodes. Each node contains two fields, called links, that are references to the previous and to the next node in the sequence of nodes. The beginning and ending nodes’ previous and next links, respectively, point to some kind of terminator, typically a sentinel node or null, to facilitate the traversal of the list. If there is only one sentinel node, then the list is circularly linked via the sentinel node. It can be conceptualized as two singly linked lists formed from the same data items, but in opposite sequential orders.

The two node links allow traversal of the list in either direction. While adding or removing a node in a doubly linked list requires changing more links than the same operations on a singly linked list, the operations are simpler and potentially more efficient (for nodes other than first nodes) because there is no need to keep track of the previous node during traversal or no need to traverse the list to find the previous node, so that its link can be modified.

Pseudocode for Basic Operations

Insert

Add(value)
  Pre: value is the value to add to the list
  Post: value has been placed at the tail of the list
  n ← node(value)
  if head = ø
    head ← n
    tail ← n
  else
    n.previous ← tail
    tail.next ← n
    tail ← n
  end if
end Add

Delete

Remove(head, value)
  Pre: head is the head node in the list
       value is the value to remove from the list
  Post: value is removed from the list, true; otherwise false
  if head = ø
    return false
  end if
  if value = head.value
    if head = tail
      head ← ø
      tail ← ø
    else
      head ← head.next
      head.previous ← ø
    end if
    return true
  end if
  n ← head.next
  while n != ø and value !== n.value
    n ← n.next
  end while
  if n = tail
    tail ← tail.previous
    tail.next ← ø
    return true
  else if n != ø
    n.previous.next ← n.next
    n.next.previous ← n.previous
    return true
  end if
  return false
end Remove

Reverse Traversal

ReverseTraversal(tail)
  Pre: tail is the node of the list to traverse
  Post: the list has been traversed in reverse order
  n ← tail
  while n != ø
    yield n.value
    n ← n.previous
  end while
end Reverse Traversal

Complexities

Time Complexity

Space Complexity

O(n)

References

• Wikipedia
• YouTube