Data Structures and Algorithm Analysis - Computer Science at ...

102 Chap. 4 Lists, Stacks, and Queuesheadcurrtailhead20 23 12 15(a)currtail2023101215(b)Figure 4.5 Illustration of a faulty linked-list implementation where curr pointsdirectly to the current node. (a) Linked list prior to inserting element withvalue 10. (b) Desired effect of inserting element with value 10.constructor the same for both variants. Because the linked list class does not needto declare a fixed-size array when the list is created, this parameter is unnecessaryfor linked lists. It is ignored by the implementation.A key design decision for the linked list implementation is how to representthe current position. The most reasonable choices appear to be a pointer to thecurrent element. But there is a big advantage to making curr point to the elementpreceding the current element.Figure 4.5(a) shows the list’s curr pointer pointing to the current element. Thevertical line between the nodes containing 23 and 12 indicates the logical positionof the current element. Consider what happens if we wish to insert a new node withvalue 10 into the list. The result should be as shown in Figure 4.5(b). However,there is a problem. To “splice” the list node containing the new element into thelist, the list node storing 23 must have its next pointer changed to point to the newnode. Unfortunately, there is no convenient access to the node preceding the onepointed to by curr.There is an easy solution to this problem. If we set curr to point directly tothe preceding element, there is no difficulty in adding a new element after curr.Figure 4.6 shows how the list looks when pointer variable curr is set to point to thenode preceding the physical current node. See Exercise 4.5 for further discussionof why making curr point directly to the current element fails.We encounter a number of potential special cases when the list is empty, orwhen the current position is at an end of the list. In particular, when the list is emptywe have no element for head, tail, and curr to point to. Implementing specialcases for insert and remove increases code complexity, making it harder tounderstand, and thus increases the chance of introducing a programming bug.These special cases can be eliminated by implementing linked lists with anadditional header node as the first node of the list. This header node is a link