Thuật toán Algorithms (Phần 20)

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Thuật toán Algorithms (Phần 20)

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  1. ELEMENTARY SEARCHING METHODS The call treeprint(head7.r) will print out the keys of the tree in order. This defines a sorting method which is remarkably similar to Quicksort, with the node at the root of the tree playing a role similar to that of the partitioning element in Quicksort. A major difference is that the tree-sorting method must use extra memory for the links, while Quicksort sorts with only a little extra memory. The running times of algorithms on binary search trees are quite depen- dent on the shapes of the trees. In the best case, the tree could be shaped like that given above for describing the comparison structure for binary search, with about lg N nodes between the root and each external node. We might, roughly, expect logarithmic search times on the average because the first ele- ment inserted becomes the root of the tree; if N keys are to be inserted at random, then this element would divide the keys in half (on the average), leading to logarithmic search times (using the same argument on the subtrees). Indeed, were it not for the equal keys, it could happen that the tree given above for describing the comparison structure for binary search would be built. This would be the best case of the algorithm, with guaranteed logarithmic running time for all searches. Actually, the root is equally likely to be any key in a truly random situation, so such a perfectly balanced tree would be extremely rare. But if random keys are inserted, it turns out that the trees are nicely balanced. The average number of steps for a treesearch in a tree built by successive insertion of N random keys is proportional to 2 In N. On the other hand, binary tree searching is susceptible to the same worst- case problems as Quicksort. For example, when the keys are inserted in order (or in reverse order) the binary tree search method is no better than the sequential search method that we saw at the beginning of this chapter. In the next chapter, we’ll examine a technique for eliminating this worst case and making all trees look more like the best-case tree. The implementations given above for the fundamental search, insert, and sort functions using binary tree structures are quite straightforward. However, binary trees also provide a good example of a recurrent theme in the study of searching algorithms: the delete function is often quite cumbersome to implement. To delete a node from a binary tree is easy if the node has no sons, like L or P in the tree above (lop it off by making the appropriate link in its father null); or if it has just one son, like G or R in the tree above (move the link in the son to the appropriate father link); but what about nodes with two sons, such as H or S in the tree above? Suppose that x is a link to such a node. One way to delete the node pointed to by x is to first set y to the node with the next highest key. By examining the treeprint routine, one can become convinced that this node must have a null left link, and that it can be found by y:=xf.r; while yt.lz do p:=yt.l. Now the deletion can be accomplished by copying yfkey and yf.info into xt.key and xt.info, then
  2. 184 CIfAPTER 14 deleting the node pointed to by y. Thus, we delete H in the example above by copying I into H, then deleting I; and we delete the E at node 3 by copying the E at node 11 into node 3, then deleting node 11. A full implementation of a treedelete procedure according to this description involves a fair amount of code to cover all the cases: we’ll forego the details because we’ll be doing similar, but more complicated manipulations on trees in the next chapter. It is quite typical for searching algorithms to require significantly more complicated implementations for deletion: the keys themselves tend to be integral to the structure, and removal of a key can involve complicated repairs. Indirect Binary Search Trees As we saw with heaps in Chapter 11, for many applications we want a search- ing structure to simply help us find records, without moving them around. For example, we might have an array a[l..N] of records with keys, and we would like the search routine to give us the index into that array of the record matching a certain key. Or we might want to remove the record with a given index from the searching structure, but still keep it in the array for some other use. To adapt binary search trees to such a situation, we simply make the info field of the nodes the array index. Then we could eliminate the key field by having the search routines access the keys in the records directly, e.g. via an instruction like if v
  3. ELEMENTARY SEARCHING METHODS 185 way of implementing binary search trees to aid in searching large arrays of records is preferred for many applications, since it avoids the extra expense of copying keys as in the previous paragraph, and it avoids the overhead of the storage allocation mechanism implied by new. The disadvantage is that space is reserved with the record array for links which may not be in use, which could lead to problems with large arrays in a dynamic situation.
  4. 186 Exercises 1. Implement a sequential searching algorithm which averages about N/2 steps for both successful and unsuccessful search, keeping the records in a sorted array. 2. Give the order of the keys after records with the keys E A S Y Q U E S T I 0 N have been put into an intially empty table with search and insert using the self-organizing search heuristic. 3. Give a recursive implementation of binary search. 4. Suppose a[i]=2i for 1 5 i 5 N. How many table positions are examined by interpolation search during the unsuccessful search for 2k - l? 5. Draw the binary search tree that results from inserting records with the keys E A S Y Q U E S T I 0 N into an initially empty tree. 6. Write a recursive program to compute the height of a binary tree: the longest distance from the root to an external node. 7. Suppose that we have an estimate ahead of time of how often search keys are to be accessed in a binary tree. Should the keys be inserted into the tree in increasing or decreasing order of likely frequency of access? Why? 8. Give a way to modify binary tree search so that it would keep equal keys together in the tree. (If there are any other nodes in the tree with the same key as any given node, then either its father or one of its sons should have an equal key.) 9. Write a nonrecursive program to print out the keys from a binary search tree in order. 10. Use a least-squares curvefitter to find values of a and b that give the best formula of the form aN In N + bN for describing the total number of instructions executed when a binary search tree is built from N random keys.
  5. 15. Balanced Trees The binary tree algorithms of the previous section work very well for a wide variety of applications, but they do have the problem of bad worst-case performance. What’s more, as with Quicksort, it’s embarrassingly true that the bad worst case is one that’s likely to occur in practice if the person using the algorithm is not watching for it. Files already in order, files in reverse order, files with alternating large and small keys, or files with any large segment having a simple structure can cause the binary tree search algorithm to perform very badly. With Quicksort, our only recourse for improving the situation was to resort to randomness: by choosing a random partitioning element, we could rely on the laws of probability to save us from the worst case. Fortunately, for binary tree searching, we can do much better: there is a general technique that will enable us to guarantee that this worst case will not occur. This technique, called balancing, has been used as the basis for several different “balanced tree” algorithms. We’ll look closely at one such algorithm and discuss briefly how it relates to some of the other methods that are used. As will become apparent below, the implementation of balanced tree algorithms is certainly a case of “easier said than done.” Often, the general concept behind an algorithm is easily described, but an implementation is a morass of special and symmetric cases. Not only is the program developed in this chapter an important searching method, but also it is a nice illustration of the relationship between a “high-level” algorithm description and a “low- level” Pascal program to implement the algorithm. Top-Down 2-3-4 Trees To eliminate the worst case for binary search trees, we’ll need some flexibility in the data structures that we use. To get this flexibility, let’s assume that we can have nodes in our trees that can hold more than one key. Specifically, we’ll 187
  6. 188 CHAPTER 15 allow J-nodes and d-nodes, which can hold two and three keys respectively. A 3-node has t.hree links coming out of it, one for all records with keys smaller than both its keys, one for all records with keys in between its two keys, and one for all records with keys larger than both its keys. Similarly, a 4-node has four links coming out of it, one for each of the intervals defined by its three keys. (The nodes in a standard binary search tree could thus be called ,%nodes: one key, two links.) We’ll see below some efficient ways of defining and implementing the basic operations on these extended nodes; for now, let’s assume we can manipulate them conveniently and see how they can be put together to form trees. For example, below is a &Y-4 tree which contains some keys from our searching example. It is easy to see how to search in such a tree. For example, to search for 0 in the tree above, we would follow the middle link from the root, since 0 is between E and R then terminate the unsuccessful search at the right link from the node containing H and I. To insert a new node in a 2-3-4 tree, we would like to do an unsuccessful search and then hook the node on, as before. It is easy to see what to if the node at which the search terminates is a 2-node: just turn it into a 3-node. Similarly, a 3-node can easily be turned into a 4-node. But what should we do if we need to insert our new node into a 4-node? The answer is that we should first split the 4-node into two 2-nodes and pass one of its keys further up in the tree. To see exactly how to do this, let’s consider what happens when the keys from A S E A R C H I N G E X A M P L E are inserted into an initially empty tree. We start out with a a-node, then a 3-node, then a 4-node: Now we need to put a second A into the 4-node. But notice that as far as the search procedure is concerned, the 4-node at the right above is exactly equivalent to the binary tree:
  7. BALANCED TREES 189 Feii E A s If our algorithm “splits” the 4-node to make this binary tree before trying to insert the A, then there will be room for A at the bottom: F5b-l E AA s Now R, C, and the H can be inserted, but when it’s time for I to be inserted, there’s no room in the 4-node at the right: Again, this 4-node must be split into two 2-nodes to make room for the I, but this time the extra key needs to be inserted into the father, changing it from a 2-node to a S-node. Then the N can be inserted with no splits, then the G causes another split, turning the root into a 4-node: But what if we were to need to split a 4-node whose father is also a 4-node? One method would be to split the father also, but this could keep happening all the way back up the tree. An easier way is to make sure that the father of any node we see won’t be a 4-node by splitting any 4-node we see on the way down the tree. For example, when E is inserted, the tree above first becomes
  8. 190 This ensures that we could handle the situation at the bottom even if E were to go into a 4-node (for example, if we were inserting another A instead). Now, the insertion of E, X, A, M, P, L, and E finally leads to the tree: The above example shows that we can easily insert new nodes into 2-3- 4 trees by doing a search and splitting 4-nodes on the way down the tree. Specifically, every time we encounter a 2-node connected to a 4-node, we should transform it into a 3-node connected to two 2-nodes: and every time we encounter a 3-node connected to a 4-node, we should transform it into a 4-node connected to two 2-nodes:
  9. BALANCED TREES These transformations are purely “local”: no part of the tree need be examined or modified other than what is diagrammed. Each of the transformations passes up one of the keys from a 4-node to its father in the tree, restructuring links accordingly. Note that we don’t have to worry explicitly about the father being a 4-node since our transformations ensure that as we pass through each node in the tree, we come out on a node that is not a 4-node. In particular, when we come out the bottom of the tree, we are not on a 4-node, and we can directly insert the new node either by transforming a 2-node to a 3-node or a 3-node to a 4-node. Actually, it is convenient to treat the insertion as a split of an imaginary 4-node at the bottom which passes up the new key to be inserted. Whenever the root of the tree becomes a 4-node, we’ll split it into three 2-nodes, as we did for our first node split in the example above. This (and only this) makes the tree grow one level “higher.” The algorithm sketched in the previous paragraph gives a way to do searches and insertions in 2-3-4 trees; since the 4-nodes are split up on the way from the top down, the trees are called top-down 2-S-4 trees. What’s interesting is that, even though we haven’t been worrying about balancing at all, the resulting trees are perfectly balanced! The distance from the root to every external node is the same, which implies that the time required by a search or an insertion is always proportional to log N. The proof that the trees are always perfectly balanced is simple: the transformations that we perform have no effect on the distance from any node to the root, except when we split the root, and in this case the distance from all nodes to the root is increased by one. The description given above is sufficient to define an algorithm for search- ing using binary trees which has guaranteed worst-case performance. However, we are only halfway towards an actual implementation. While it would be possible to write algorithms which actually perform transformations on dis- tinct data types representing 2-, 3-, and 4-nodes, most of the things that need to be done are very inconvenient in this direct representation. (One can be- come convinced of this by trying to implement even the simpler of the two node transformations.) Furthermore, the overhead incurred in manipulating the more complex node structures is likely to make the algorithms slower than standard binary tree search. The primary purpose of balancing is to provide “insurance” against a bad worst case, but it would be unfortunate to have to pay the overhead cost for that insurance on every run of the algorithm. Fortunately, as we’ll see below, there is a relatively simple representation of 2-, 3-, and 4-nodes that allows the transformations to be done in a uniform way with very little overhead beyond the costs incurred by standard binary tree search.
  10. 192 CHAPTER 15 Red-Black Trees Remarkably, it is possible to represent 2-3-4 trees as standard binary trees (2-nodes only) by using only one extra bit per node. The idea is to represent 3-nodes and 4nodes as small binary trees bound together by “red” links which contrast with the “black” links which bind the 2-3-4 tree together. The representation is simple: 4-nodes are represented as three 2-nodes connected by red links and 3-nodes are represented as two 2-nodes connected by a red link (red links are drawn as double lines): (Either orientation for a 3-node is legal.) The binary tree drawn below is one way to represent the final tree from the example above. If we eliminate the red links and collapse the nodes they connect together, the result is the 2-3-4 tree from above. The extra bit per node is used to store the color of the link pointing to that node: we’ll refer to 2-3-4 trees represented in this way as red-black trees.
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