A Practical Introduction to Data Structures and Algorithm Analysis

116 Chap. 4 Lists, Stacks, and QueuesIn Figure 4.11, you should note the use of the static definition for the freelistheader. The purpose of the keyword static is to create a single variable sharedamong all instances of the Link nodes. We want only a single freelist for all Linknodes of a given type. A program might create multiple lists. If they are all of thesame type (that is, their element types are the same), then they can and should sharethe same freelist. This will happen with the implementation of Figure 4.11. If listsare created that have different element types, because this code is implementedwith a generic, the need for different list implementations will be discovered bythe runtime environment. Separate versions of the list class will be generated foreach element type. Thus, each element type will also get its own separate copy ofthe Link class. And each distinct Link class implementation will get a separatefreelist.Let’s consider a more complex situation where we want separate freelists, butwe don’t know this until runtime. For example, perhaps we are using lists to storevariable-length strings. A given node gets space allocated for a string of a specificlength. We’d like to be able to reuse the nodes by placing them on a freelist, insteadof relying on the system free store operators. But we cannot reuse a given list nodeunless the size allocated for its string matches the length of the string we want tostore in this node. Thus, each specific node size must be stored on its own freelist.That is, the nodes for strings of length one are stored on one freelist, the nodes forstrings of length two on another freelist, and so on. In this way, its easy to find anode of the proper size.Unfortunately, we don’t know in advance what the various sizes of strings willbe. If we want to support long strings, then there could be thousands of differentnode sizes, many of which are not used in any particular run of the program. Thus,we do not want to allocate freelists in advance for each potential node length. Onthe other hand, we need to make sure no more than one copy of the freelist for agiven size is created.We can modify our freelist free-store operations to request the appropriate freelist.This access function will search for the proper freelist. If it exists, that freelistis used. If not, that freelist is created.4.1.3 Comparison of List ImplementationsNow that you have seen two substantially different implementations for lists, it isnatural to ask which is better. In particular, if you must implement a list for sometask, which implementation should you choose?Array-based lists have the disadvantage that their size must be predeterminedbefore the array can be allocated. Array-based lists cannot grow beyond their pre-