A doubly linked list allows bidirectional traversal by storing two pointers per node in Java

Painting Code 39 Full ASCII in Java A doubly linked list allows bidirectional traversal by storing two pointers per node
A doubly linked list allows bidirectional traversal by storing two pointers per node
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Figure 1718 A doubly linked list
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Figure 1719 An empty doubly linked list
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Symmetry demands that we use both a head and a t a i l and that we support roughly twice as many operations
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First, an empty list now consists of a head and tail, connected as shown in Figure 1719 Note that head->prev and tail->next are not needed in the algorithms and are not even initialized The test for emptiness is now
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When we advance past the end of the list, we now hit the t a i l node instead of
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We no longer use NULL to decide whether an advance has taken us past the end of the list Instead, we have gone past the end if current is either head or tail (recall that we can go in either direction) The retreat operation can be implemented by
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Insertion and removal involve twice as many pointer changes as for a singly linked list
Before describing some of the additional operations that are available, let us consider how the insertion and removal operations change Naturally, we can now do both insertBefore and insertAfterTwice as many pointer moves are involved for insertAfter with doubly linked lists as with singly linked lists If we write each statement explicitly, we obtain
newNode = new Node( x ) ; newNode->prev = current; newNode->next = current->next; newNode->prev->next = newNode; newNode->next->prev = newNode; current = newNode:
// // // //
Set Set Set Set
x's prev x ' s next a ' s next b ' s prev
pointer pointer pointer pointer
As we showed earlier, the first two pointer moves can be collapsed into the Node construction that is done by newThe changes (in order I , 2, 3, 4) are illustrated in Figure 1720
Doubly Linked Lists
a n d u l z r ~ lkn
Figure 1720 Insertion in a doubly linked list by getting new node and then changing pointers in the order indicated
Figure 1720 can also be used as a guide in the removal algorithm Unlike the singly linked list, we can remove the current node because the previous node is available to us automatically Thus to remove x we have to change a's next pointer and b's p r e v pointer The basic moves are
oldNode = current; oldNode->prev->next = oldNode->next;//Set a ' s next pointer oldNode->next->prev = oldNode->prev;// Set b ' s prev pointer delete oldNode; current = head; l i So current is not stale
The remove operation can proceed from the current node because we can obtain the previous node instantly
To do a complete doubly linked list implementation, we need to decide which operations to support We can reasonably expect twice as many operations as in the singly linked list Each individual procedure is similar to the linked list routines; only the dynamic operations involve additional pointer moves Moreover, for many of the routines, the code is dominated by error checks Although some of the checks will change (eg, we do not test against NULL), they certainly do not become any more complex In Section 175, we use a doubly linked list to implement the STL list class, along with its associated iterators There are lots of routines, but most are short A popular convention is to create a circularly linked list, in which the last cell's next pointer points back to f i r s t , which can be done with or without a header Typically, i t is done without a header because the header's main purpose is to ensure that every node has a previous node, which is already true for a nonempty circularly linked list Without a header, we have only the empty list as a special case We maintain a pointer to the first node, but that is not the same as a header node We can use circularly linked lists and doubly linked lists simultaneously, as shown in Figure 172 1 The circular list is useful when we want searching to allow wraparound, as is the case for some text editors In Exercise 1720 you are asked to implement a circularly and doubly linked list
In a circularly linked list, the last cell's n e x t pointer points to f i r s t T h i s action is useful when wraparound matters