19+-+Templates - Linked Lists Double-ended list EECS 280...

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11/22/2009 1 Templates EECS 280 Programming and Introductory Data Structures 1 Linked Lists Double-ended list 2 circle5 What if we wanted to insert something at the end of the list? circle5 Intuitively, with the current representation, we'd need to walk down the list until we found "the last element", and then insert it there. circle5 That's not very efficient, because we'd have to examine every element to insert anything at the tail. circle5 Instead, we'll change our concrete representation to track both the front and the back of our list. first Linked Lists Double-ended list 3 circle5 The new representational invariant has two node pointers: class IntList { node *first; node *last; public: }; circle5 The invariant on first is unchanged. circle5 The invariant on "last" is: circle5 last points to the last node of the list if it is not empty, and is NULL otherwise. Linked Lists Double-ended list 4 circle5 So, in an empty list, both data members point to NULL. circle5 However, if the list is non-empty, they look like this: circle5 Note: Adding this new data member requires that every method (except isEmpty ) be re-written. circle5 In lecture, we'll only write insertLast . first last Linked Lists Double-ended list 5 circle5 First, we create the new node, and establish its invariants: void IntList::insertLast(int v) { node *np = new node; np->next = NULL; np->value = v; ... } Linked Lists Double-ended list 6 circle5 To actually insert, there are two cases: circle5 If the list is empty, we need to reestablish the invariants on first and last (the new node is both the first and last node of the list) circle5 If the list is not empty, there are two broken invariants. The "old" last->next element (incorrectly) points to NULL, and the last field no longer points to the last element. first last np
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11/22/2009 2 Linked Lists Double-ended list 7 void IntList::insertLast(int v) { node *np = new node; np->next = NULL; np->value = v; if (isEmpty()) { first = last = np; } else { last->next = np; last = np; } } first last np Linked Lists Double-ended list 8 circle5 This is efficient, but only for insertion. circle5 Question : Why is removal from the end expensive? first last np first last Linked Lists Double-ended list 9 circle5 To make removal from the end efficient, as well, we have to have a “doubly-linked” list, so we can go forward and backward. circle5 To do this, we're going to change the representation yet again. circle5 In our new representation, a node is: struct node { node *next; node *prev; int value; } circle5 The next and value fields stay the same.
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