Thuật toán Algorithms (Phần 43)

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WEIGHTED GRAPHS 413 priority-first search method will be faster for some graphs, Prim’s for some others, Kruskal’s for still others. As mentioned above, the worst case for the priority-first search method is (E + V)logV while the worst case for Prim’s is V2 and the worst case for Kruskal’s is Elog E. But it is unwise to choose between the algorithms on the basis of these formulas because “worstrcase” graphs are unlikely to occur in practice. In fact, the priority-first search method and Kruskal’s method are both likely to run in time proportional to E for graphs that arise in practice: the first because most edges do not really require a priority queue adjustment that takes 1ogV steps and the second because the longest edge in the minimum spanning tree is probably sufficiently short that not many edges are taken off the priority queue. Of course, Prim’s method also runs in time proportional to about E for dense graphs (but it shouldn’t be used for sparse graphs). Shortest Path The shortest path problem is to find the path in a weighted graph connecting two given vertices x and y with the property that the sum of the weights of all the edges is minimized over all such paths. If the weights are all 1, then the problem is still interesting: it is to find the path containing the minimum number of edges which connects x and y. Moreover, we’ve already considered an algorithm which solves the problem: breadth-first search. It is easy to prove by induction that breadth-first search starting at x will first visit all vertices which can be reached from z with 1 edge, then al: vertices which can be reached from x with 2 edges, etc., visiting all vertices which can be reached with k edges before encountering any that require k + 1 edges. Thus, when y is first encountered, the shortest path from x has been found (because no shorter paths reached y). In general, the path from z to y could touch all the vertices, so we usually consider the problem of finding the shortest paths connecting a given vertex x with each of the other vertices in the graph. Again, it turns out that the problem is simple to solve with the priority graph traversal algorithm of the previous chapter. If we draw the shortest path from x to each other vertex in the graph, then we clearly get no cycles, and we have a spanning tree. Each vertex leads to a different spanning tree; for example, the following three diagrams show the shortest path spanning trees for vertices A, B, and E in the example graph that we’ve been using.
414 CHAPTER 31 The priority-first search solution to this problem is very similar to the solution for the minimum spanning tree: we build the tree for vertex z by adding, at each step, the vertex on the fringe which is closest to z (before, we added the one closest to the tree). To find which fringe vertex is closest to CC, we use the val array: for each tree vertex k, val[k] will be the distance from that vertex to z, using the shortest path (which must be comprised of tree nodes). When k is added to the tree, we update the fringe by going through k’s adjacency list. For each node t on the list, the shortest distance to z through k from tf.v is val[k]+tf.weight. Thus, the algorithm is trivially implemented by using this quantity for priority in the priority graph traversal program. The following sequence of diagrams shows the construction of the shortest path search tree for vertex A in our example. b-0 1 B'C A 1 B'C k 0 2 F 2 D
WEIGHTED GRAPHS 415 First we visit the closest vertex to A, which is B. Then both C and F are distance 2 from A, so we visit them next (in whatever order the priority queue returns them). Then D can be attached at F or at B to get a path of distance 3 to A. (The algorithm attaches it to B because it was put on the tree before F, so D was already on the fringe when F was put on the tree and F didn’t provide a shorter path to A.) Finally, E and G are visited. As usual, the tree is represented by the dad array of father links. The following table shows the array computed by the priority graph traversal procedure for our example: A B C D E F G dad: A B B F A E val: 0 1 2 3 4 2 5 Thus the shortest path from A to G has a total weight of 5 (found in the val entry for G) and goes from A to F to E to G (found by tracing backwards in the dad array, starting at G). Note that the correct operation of this program depends on the val entry for the root being zero, the convention that we adopted for sparsepfs. As before, the priority graph traversal algorithm has a worst-case running time proportional to (E + V) log V, though a different implementation of the priority queue can give a V2 algorithm, which is appropriate for dense graphs. Below, we’ll examine this implementation of priority graph traversal for dense graphs in full detail. For the shortest path problem, this reduces to a method discovered by E. Dijkstra in 1956. Though the methods are the same in essence, we’ll refer to the sparsepfs program of the previous chapter with priority replaced by val [k] + tt . weight as the “priority-first search solution” to the shortest paths problem and the adjacency matrix version given below as “Dijkstra’s algorithm.” Dense Graphs As we’ve discussed, when a graph is represented with a adjacency matrix, it is best to use an unordered array representation for the priority queue in order to achieve a V2 running time for any priority graph traversal algorithm. That is, this provides a linear algorithm for the priority first search (and thus the minimum spanning tree and shortest path problems) for dense graphs. Specifically, we maintain the priority queue in the val array just as in sparsepfs but we implement the priority queue operations directly rather than using heaps. First, we adopt the convention that the priority values in the val array will be negated, so that the sign of a val entry tells whether the corresponding vertex is on the tree or the priority queue. To change the
416 CRAPTER 31 priority of a vertex, we simply assign the new priority to the vaJ entry for that vertex. To remove the highest priority vertex, we simply scan through the vaJ array to find the vertex with the largest negative (closest to 0) vaJ value (then complement its vaJ entry). After making these mechanical changes to the sparsepfs program of the previous chapter, we are left with the following compact program. procedure densepfs; var k, min, t: integer; begin for k:=l to Vdo begin vaJ[k]:=-unseen; dad[k]:=O end; vaJ[O]:=-(unseen+l); min:=l; repeat k : = m i n ; vaJ[k]:=-vaJ[k]; min:=O; if vaJ[k]=unseen then vaJ[k] :=O; for t:=l to Vdo if vaJ[t]
WEIGHTED GRAPHS 417 instead of indirect heap). These changes yield a worst-case running time proportional to V2, as opposed to (E + V)logV for sparsepfs. That is, the running time is linear for dense graphs (when E is proportional to V2), but sparsepfs is likely to be much faster for sparse graphs. Geometric Problems Suppose that we are given N points in the plane and we want to find the shortest set of lines connecting all the points. This is a geometric problem, called the Euclidean minimum spanning tree problem. It can be solved us- ing the graph algorithm given above, but it seems clear that the geometry provides enough extra structure to allow much more efficient algorithms to be developed. The way to solve the Euclidean problem using the algorithm given above is to build a complete graph with N vertices and N(N - 1)/2 edges, one edge connecting each pair of vertices weighted with the distance between the corresponding points. Then the minimum spanning tree can be found with the algorithm above for dense graphs in time proportional to N2. It has been proven that it, is possible to do better. The point is that the geometric structure makes most of the edges in the complete graph irrelevant to the problem, and we can eliminate most of the edges before even starting to construct the minimum spanning tree. In fact, it has been proven that the minimum spanning tree is a subset of the graph derived by taking only the edges from the dual of the Voronoi diagram (see Chapter 28). We know that this graph has a number of edges proportional to N, and both Kruskal’s algorithm and the priority-first search method work efficiently on such sparse graphs. In principle, then, we could compute the Voronoi dual (which takes time proportional to Nlog N), then run either Kruskal’s algorithm or the priority-first search method to get a Euclidean minimum spanning tree algo- rithm which runs in time proportional to N log N. But writing a program to compute the Voronoi dual is quite a challenge even for an experienced programmer. Another approach which can be used for random point sets is to take advantage of the distribution of the points to limit the number of edges included in the graph, as in the grid method used in Chapter 26 for range searching. If we divide up the plane into squares such that each square is likely to contain about 5 points, and then include in the graph only the edges connecting each point to the points in the neighboring squares, then we are very likely (though not guaranteed) to get all the edges in the minimum spanning tree, which would mean that Kruskal’s algorithm or the priority-first search method would efficiently finish the job. It is interesting to reflect on the relationship between graph and geometric algorithms brought out by the problem posed in the previous paragraphs. It
CHAPTER 31 is certainly true that many problems can be formulated either as geometric problems or as graph problems. If the actual physical placement of objects is a dominating characteristic, then the geometric algorithms of the previous section may be appropriate, but if interconnections between objects are of fundamental importance, then the graph algorithms of this section may be better. The Euclidean minimum spanning tree seems to fall at the interface between these two approaches (the input involves geometry and the output involves interconnections) and the development of simple, straightforward methods for this and related problems remains an important though elusive goal. Another example of the interaction between geometric and graph algo- rithms is the problem of finding the shortest path from z to y in a graph whose vertices are points in the plane and whose edges are lines connecting the points. For example, the maze graph at the end of Chapter 29 might be viewed as such a graph. The solution to this problem is simple: use priority first searching, setting the priority of each fringe vertex encountered to the distance in the tree from z to the fringe vertex (as in the algorithm given) plus the Euclidean distance from the fringe vertex to y. Then we stop when y is added to the tree. This method will very quickly find the shortest path from x to y by always going towards y, while the standard graph algorithm has to “search” for y. Going from one corner to another of a large maze graph like that one at the end of Chapter 29 might require examining a number of nodes proportional to @, while the standard algorithm has to examine virtually all the nodes.
WEIGHTED GRAPHS 419 Exercises 1. Give another minimum spanning tree for the example graph at the begin- ning of the chapter. 2. Give an algorithm to find the minimum spanning forest of a connected graph (each vertex must be touched by some edge, but the resulting graph doesn’t have to be connected). 3. Is there a graph with V vertices and E edges for which the priority-first solution to the minimum spanning tree problem algorithm could require time proportional to (E + V) log V? Give an example or explain your answer. 4. Suppose we maintained the priority queue as a sorted list in the general graph traversal implementations. What would be the worst-case run- ning time, to within a constant factor? When would this method be appropriate, if at all? 5. Give counterexamples which show why the following “greedy” strategy doesn’t work for either the shortest path or the minimum spanning tree problems: “at each step visit the unvisited vertex closest to the one just visited.” 6. Give the shortest path trees for the other nodes in the example graph. 7. Find the shortest path from the upper right-hand corner to the lower left-hand corner in the maze graph of Chapter 29, assuming all edges have weight 1. 8. Write a program to generate random connected graphs with V vertices, then find the minimum spanning tree and shortest path tree for some vertex. Use random weights between 1 and V. How do the weights of the trees compare for different values of V? 9. Write a program to generate random complete weighted graphs with V vertices by simply filling in an adjacency matrix with random numbers be- tween 1 and V. Run empirical tests to determine which method finds the minimum spanning tree faster for V = 10, 25, 100: Prim’s or Kruskal’s. 10. Give a counterexample to show why the following method for finding the Euclidean minimum spanning tree doesn’t work: “Sort the points on their x coordinates, then find the minimum spanning trees of the first half and the second half, then find the shortest edge that connects them.”
32. Directed Graphs Directed graphs are graphs in which edges connecting nodes are one- way; this added structure makes it more difficult to determine various properties. Processing such graphs is akin to traveling around in a city with many one-way streets or to traveling around in a country where airlines rarely run round-trip routes: getting from one point to another in such situations can be a challenge indeed. Often the edge direction reflects some type of precedence relationship in the application being modeled. For example, a directed graph might be used to model a manufacturing line, with nodes corresponding to jobs to be done and with an edge from node x to node y if the job corresponding to node x must be done before the job corresponding to node y. How do we decide when to perform each of the jobs so that none of these precedence relationships are violated? In this chapter, we’ll look at depth-first search for directed graphs, as well as algorithms for computing the transitive closure (which summarizes con- nectivity information) and for topological sorting and for computing strongly connected components (which have to do with precedence relationships). As mentioned in Chapter 29, representations for directed graphs are simple extensions of representations for undirected graphs. In the adjacency list representation, each edge appears only once: the edge from z to y is represented as a list node containing y in the linked list corresponding to x. In the adjacency matrix representation, we need to maintain a full V-by-V matrix, with a 1 bit in row x and column y (but not necessarily in row y and column Z) if there is an edge from x to y. A directed graph similar to the undirected graph that we’ve been con- sidering is drawn below. This graph consists of the edges AG AI3 CA LM JM JLJKEDDFHIFEAFGEGCHGGJLGMML. 421
422 CHAPTER 32 The order in which the edges appear is now significant: the notation AG describes an edge which points from A to G, but not from G to A. But it is possible to have two edges between two nodes, one in either direction (we have both HI and IH and both LM and ML in the above graph). Note that, in these representations, no difference could be perceived between an undirected graph and a directed graph with two opposite directed edges for each edge in the undirected graph. Thus, some of algorithms in this chapter can be considered generalizations of algorithms in previous chapters. Depth-First Search The depth-first search algorithm of Chapter 29 works properly for directed graphs exactly as given. In fact, its operation is a little more straightforward than for undirected graphs because we don’t have to be concerned with double edges between nodes unless they’re explicitly included in the graph. However, the search trees have a somewhat more complicated structure. For example, the following depth-first search structure describes the operation of the recur- sive algorithm of Chapter 29 on our sample graph. As before, this is a redrawn version of the graph, with solid edges correspond-