DOCSLIB.ORG

Graph Traversal

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Graph Traversal GRAPH TRAVERSAL Lecture 18 CS 2110 — Spring 2019 JavaHyperText Topics 2 “Graphs”, topic 8: DFS, BFS Graph Representations 3 1 2 Adjacency list 3 4 Adjacency matrix 1 2 3 0 1 2 3 4 0 F F F F F 2 4 1 F F T T F 2 F F F F T 3 2 4 3 F F T F T 4 4 F F F F F 4 Review: Sparse vs. Dense (improved definitions in Lec 17) Graph Search 5 Graph Traversal 6 Goal: visit each vertex 2 3 that is reachable from 1 4 some starting vertex 7 5 6 8 And: even if there are 2 3 many paths to a node, 1 visit only once 4 7 5 6 Two algorithms: DFS, BFS 8 7 Depth-First Search (DFS) Intuition: one person exploring a maze Depth-First Search 8 Idea: Recursively visit each unvisited neighbor /** Visit every node reachable along a path of unvisited nodes from node v. Precondition: v is unvisited. */ void dfs(Vertex v) { 3 1 2 mark v visited; for all edges (v,u): 4 if u is unmarked: 7 5 dfs(u); 6 8 } 8 dfs(1) visits the nodes in this order: 1, 2, 3, 5, 7, 8 9 Poll #1 Depth-First Search 10 /** Visit every node reachable along a path of unvisited nodes from node v. Precondition: v is unvisited. */ void dfs(Vertex v) { mark v visited; for all edges (v,u): if u is unmarked: dfs(u); } Demo DFS Space Efficiency 11 void dfs(Vertex v) { mark v visited; Suppose graph has for all edges (v,u): V vertices and E edges if u is unmarked: dfs(u); } 1 2 3 4 Space required? • Mark for each vertex: O(V) • Frame for each recursive call: O(V) Worst case: O(V) 8 7 6 5 DFS Time Efficiency 12 void dfs(Vertex v) { mark v visited; Suppose graph has for all edges (v,u): V vertices and E edges if u is unmarked: dfs(u); } Time required? 1 2 • Mark each vertex: O(V) • Recursive call for on each unvisited vertex: O(V) • Find each edge 4 3 • in adj list: O(E): Worst case: O(V+E) • In adj matrix: O(V2): Worst case: O(V2) Variant: Iterative DFS 13 Same algorithm; non-recursive implementation void dfs(Vertex u) { Stack s= new Stack(); 2 3 s.push(u); 1 while (s is not empty) { 4 u= s.pop(); if (u not visited) { 77 5 visit u; 6 for each edge (u, v): 88 s.push(v); } 8 } 57 } 5 u: 352178 Stack: 5312 Demo Visit order was 1, 7, 8, 5, 2, 3: differs from before because of order edges processed 14 Breadth-First Search (BFS) Intuition: Search party fanning out in all directions Breadth-First Search 15 Idea: Iteratively process the graph in "layers" moving further away from the source vertex. 3 1 2 4 77 55 6 88 Breadth-First Search 16 Idea: Iteratively process the graph in "layers" moving further away from the source vertex. /** Visit all vertices reachable on unvisited paths from u. */ 3 void dfsbfs(int u) { 1 2 StackQueue s=q= new Stack()Queue() 4 s.pushq.add(u);(u); while ( q is not empty ) {) { 77 55 u= s.popq.remove(); (); if (u not visited) { 6 visit u; 8 for each edge(u, v):(u, v): 8 s.pushq.add(v);(v); } } } } Queue: 21 5 7 3 5 8 5 } Visit order was 1, 2, 5, 7, 3, 8 Demo 17 Poll #2 Improved BFS 18 Idea: Don’t put vertex in queue if already encountered /** Visit all vertices reachable on unvisited paths from u. */ void bfs(int u) { Queue q= new Queue q.add(u); while ( q is not empty ) { u= q.remove(); if (u not visited) { visit u; for each (u, v): if (v not encountered) { mark v as encountered; q.add(v); } } } } Analyzing BFS 19 /** Visit all vertices reachable on unvisited paths from u. */ void bfs(int u) { Same as DFS. Queue q= new Queue Space: O(V) q.add(u); Time: while ( q is not empty ) { • Adj list: O(V+E) u= q.remove(); • Adj matrix: O(V2) if (u not visited) { visit u; for each (u, v): if (v not encountered) { mark v as encountered; q.add(v); } } } } Comparing Traversal Algorithms 20 DFS (recursive) DFS & BFS (iterative, improved*) 2 2 ¨ Time: O(V+E) or O(V ) ¨ Time: O(V+E) or O(V ) ¨ Space: O(V) ¨ Space: O(V) *Without improvement, space becomes O(E).
Load more

Recommended publications
  • Graph Traversals
    Graph Traversals CS200 - Graphs 1 Tree traversal reminder Pre order A A B D G H C E F I In order B C G D H B A E C F I Post order D E F G H D B E I F C A Level order G H I A B C D E F G H I Connected Components n The connected component of a node s is the largest set of nodes reachable from s. A generic algorithm for creating connected component(s): R = {s} while ∃edge(u, v) : u ∈ R∧v ∉ R add v to R n Upon termination, R is the connected component containing s. q Breadth First Search (BFS): explore in order of distance from s. q Depth First Search (DFS): explores edges from the most recently discovered node; backtracks when reaching a dead- end. 3 Graph Traversals – Depth First Search n Depth First Search starting at u DFS(u): mark u as visited and add u to R for each edge (u,v) : if v is not marked visited : DFS(v) CS200 - Graphs 4 Depth First Search A B C D E F G H I J K L M N O P CS200 - Graphs 5 Question n What determines the order in which DFS visits nodes? n The order in which a node picks its outgoing edges CS200 - Graphs 6 DepthGraph Traversalfirst search algorithm Depth First Search (DFS) dfs(in v:Vertex) mark v as visited for (each unvisited vertex u adjacent to v) dfs(u) n Need to track visited nodes n Order of visiting nodes is not completely specified q if nodes have priority, then the order may become deterministic for (each unvisited vertex u adjacent to v in priority order) n DFS applies to both directed and undirected graphs n Which graph implementation is suitable? CS200 - Graphs 7 Iterative DFS: explicit Stack dfs(in v:Vertex) s – stack for keeping track of active vertices s.push(v) mark v as visited while (!s.isEmpty()) { if (no unvisited vertices adjacent to the vertex on top of the stack) { s.pop() //backtrack else { select unvisited vertex u adjacent to vertex on top of the stack s.push(u) mark u as visited } } CS200 - Graphs 8 Breadth First Search (BFS) n Is like level order in trees A B C D n Which is a BFS traversal starting E F G H from A? A.
    [Show full text]
  • Graph Traversal with DFS/BFS
    Graph Traversal Graph Traversal with DFS/BFS One of the most fundamental graph problems is to traverse every Tyler Moore edge and vertex in a graph. For correctness, we must do the traversal in a systematic way so that CS 2123, The University of Tulsa we dont miss anything. For efficiency, we must make sure we visit each edge at most twice. Since a maze is just a graph, such an algorithm must be powerful enough to enable us to get out of an arbitrary maze. Some slides created by or adapted from Dr. Kevin Wayne. For more information see http://www.cs.princeton.edu/~wayne/kleinberg-tardos 2 / 20 Marking Vertices To Do List The key idea is that we must mark each vertex when we first visit it, We must also maintain a structure containing all the vertices we have and keep track of what have not yet completely explored. discovered but not yet completely explored. Each vertex will always be in one of the following three states: Initially, only a single start vertex is considered to be discovered. 1 undiscovered the vertex in its initial, virgin state. To completely explore a vertex, we look at each edge going out of it. 2 discovered the vertex after we have encountered it, but before we have checked out all its incident edges. For each edge which goes to an undiscovered vertex, we mark it 3 processed the vertex after we have visited all its incident edges. discovered and add it to the list of work to do. A vertex cannot be processed before we discover it, so over the course Note that regardless of what order we fetch the next vertex to of the traversal the state of each vertex progresses from undiscovered explore, each edge is considered exactly twice, when each of its to discovered to processed.
    [Show full text]
  • Graph Traversal and Linear Programs October 6, 2016
    CS 125 Section #5 Graph Traversal and Linear Programs October 6, 2016 1 Depth first search 1.1 The Algorithm Besides breadth first search, which we saw in class in relation to Dijkstra's algorithm, there is one other fundamental algorithm for searching a graph: depth first search. To better understand the need for these procedures, let us imagine the computer's view of a graph that has been input into it, in the adjacency list representation. The computer's view is fundamentally local to a specific vertex: it can examine each of the edges adjacent to a vertex in turn, by traversing its adjacency list; it can also mark vertices as visited. One way to think of these operations is to imagine exploring a dark maze with a flashlight and a piece of chalk. You are allowed to illuminate any corridor of the maze emanating from your current position, and you are also allowed to use the chalk to mark your current location in the maze as having been visited. The question is how to find your way around the maze. We now show how the depth first search allows the computer to find its way around the input graph using just these primitives. Depth first search uses a stack as the basic data structure. We start by defining a recursive procedure search (the stack is implicit in the recursive calls of search): search is invoked on a vertex v, and explores all previously unexplored vertices reachable from v. Procedure search(v) vertex v explored(v) := 1 previsit(v) for (v; w) 2 E if explored(w) = 0 then search(w) rof postvisit(v) end search Procedure DFS (G(V; E)) graph G(V; E) for each v 2 V do explored(v) := 0 rof for each v 2 V do if explored(v) = 0 then search(v) rof end DFS By modifying the procedures previsit and postvisit, we can use DFS to solve a number of important problems, as we shall see.
    [Show full text]
  • Graphs, Connectivity, and Traversals
    Graphs, Connectivity, and Traversals Definitions Like trees, graphs represent a fundamental data structure used in computer science. We often hear about cyber space as being a new frontier for mankind, and if we look at the structure of cyberspace, we see that it is structured as a graph; in other words, it consists of places (nodes), and connections between those places. Some applications of graphs include • representing electronic circuits • modeling object interactions (e.g. used in the Unified Modeling Language) • showing ordering relationships between computer programs • modeling networks and network traffic • the fact that trees are a special case of graphs, in that they are acyclic and connected graphs, and that trees are used in many fundamental data structures 1 An undirected graph G = (V; E) is a pair of sets V , E, where • V is a set of vertices, also called nodes. • E is a set of unordered pairs of vertices called edges, and are of the form (u; v), such that u; v 2 V . • if e = (u; v) is an edge, then we say that u is adjacent to v, and that e is incident with u and v. • We assume jV j = n is finite, where n is called the order of G. •j Ej = m is called the size of G. • A path P of length k in a graph is a sequence of vertices v0; v1; : : : ; vk, such that (vi; vi+1) 2 E for every 0 ≤ i ≤ k − 1. { a path is called simple iff the vertices v0; v1; : : : ; vk are all distinct.
    [Show full text]
  • Lecture 16: Dijkstra’S Algorithm (Graphs)
    CSE 373: Data Structures and Algorithms Lecture 16: Dijkstra’s Algorithm (Graphs) Instructor: Lilian de Greef Quarter: Summer 2017 Today • Announcements • Graph Traversals Continued • Remarks on DFS & BFS • Shortest paths for weighted graphs: Dijkstra’s Algorithm! Announcements: Homework 4 is out! • Due next Friday (August 4th) at 5:00pm • May choose to pair-program if you like! • Same cautions as last time apply: choose partners and when to start working wisely! • Can almost entirely complete using material by end of this lecture • Will discuss some software-design concepts next week to help you prevent some (potentially non-obvious) bugs Another midterm correction… ( & ) Bring your midterm to *any* office hours to get your point back. I will have the final exam quadruple-checked to avoid these situations! (I am so sorry) Graphs: Traversals Continued And introducing Dijkstra’s Algorithm for shortest paths! Graph Traversals: Recap & Running Time • Traversals: General Idea • Starting from one vertex, repeatedly explore adjacent vertices • Mark each vertex we visit, so we don’t process each more than once (cycles!) • Important Graph Traversal Algorithms: Depth First Search (DFS) Breadth First Search (BFS) Explore… as far as possible all neighbors first before backtracking before next level of neighbors Choose next vertex using… recursion or a stack a queue • Assuming “choose next vertex” is O(1), entire traversal is • Use graph represented with adjacency Comparison (useful for Design Decisions!) • Which one finds shortest paths? • i.e. which is better for “what is the shortest path from x to y” when there’s more than one possible path? • Which one can use less space in finding a path? • A third approach: • Iterative deepening (IDFS): • Try DFS but disallow recursion more than K levels deep • If that fails, increment K and start the entire search over • Like BFS, finds shortest paths.
    [Show full text]
  • Data Structures and Algorithms Lecture 18: Minimum Spanning Trees (Graphs)
    CSE 373: Data Structures and Algorithms Lecture 18: Minimum Spanning Trees (Graphs) Instructor: Lilian de Greef Quarter: Summer 2017 Today • Spanning Trees • Approach #1: DFS • Approach #2: Add acyclic edges • Minimum Spanning Trees • Prim’s Algorithm • Kruskal’s Algorithm Announcements • Midterms • I brought midterms with me, can get them after class • Next week, will only have them at CSE220 office hours • Reminder: hw4 due on Friday! Spanning Trees & Minimum Spanning Trees For undirected graphs Introductory Example All the roads in Seattle are covered in snow. You were asked to shovel or plow snow from roads so that Seattle drivers can travel. Because you don’t want to shovel/plow that many roads, what is the smallest set of roads to clear in order to reconnect Seattle? Spanning Trees • Goal: Given a connected undirected graph G=(V,E), find a minimal subset of edges such that G is still connected • A graph G2 = (V,E2) such that G2 is connected and removing any edge from E2 makes G2 disconnected Observations 1. Any solution to this problem is a tree • Recall a tree does not need a root; just means acyclic • For any cycle, could remove an edge and still be connected 2. Solution not unique unless original graph was already a tree 3. Problem ill-defined if original graph not connected • So |E| >= |V|-1 4. A tree with |V| nodes has edges • So every solution to the spanning tree problem has edges Two Approaches Different algorithmic approaches to the spanning-tree problem: 1. Do a graph traversal (e.g., depth-first search, but any traversal will do), keeping track of edges that form a tree 2.
    [Show full text]
  • 9 Graph Traversal
    9 Graph Traversal Suppose you are working in the traffic planning department of a town with a nice medieval center.1 An unholy coalition of shop owners, who want more street-side parking, and the Green Party, which wants to discourage car traffic altogether, has decided to turn most streets into one-way streets. You want to avoid the worst by checking whether the current plan maintains the minimum requirement that one can still drive from every point in town to every other point. In the language of graphs (see Sect. 2.12), the question is whether the directed graph formed by the streets is strongly connected. The same problem comes up in other applications. For example, in the case of a communication network with unidirec- tional channels (e.g., radio transmitters), we want to know who can communicate with whom. Bidirectional communication is possible within the strongly connected components of the graph. We shall present a simple, efficient algorithm for computing strongly connected components (SCCs) in Sect. 9.3.2. Computing SCCs and many other fundamental problems on graphs can be reduced to systematic graph exploration, inspecting each edge exactly once. We shall present the two most important exploration strategies: breadth-first search (BFS) in Sect. 9.1 and depth-first search (DFS) in Sect. 9.3. Both strategies construct forests and partition the edges into four classes: Tree edges com- prising the forest, forward edges running parallel to paths of tree edges, backward edges running antiparallel to paths of tree edges, and cross edges. Cross edges are all remaining edges; they connect two different subtrees in the forest.
    [Show full text]
  • 10 Graph Algorithms
    Ghosh: Distributed Systems Chapter 10: Graph Algorithms 10 Graph Algorithms 10.1. Introduction The topology of a distributed system is represented by a graph where the nodes represent processes, and the links represent communication channels. Accordingly, distributed algorithms for various graph theoretic problems have numerous applications in communication and networking. Here are some motivating examples. Our first example deals with routing in a communication network. When a message is sent from node i to a non-neighboring node j, the intermediate nodes route the message based on the information stored in the local routing table, so that eventually the message reaches the destination node. This is called destination-based routing. An important problem is to compute these routing tables and maintain them, so that messages reach their destinations in smallest number of hops or with minimum delay. The minimum hop routing is equivalent to computing the shortest path between a pair of nodes using locally available information. Our second example focuses on the amount of space required to store a routing table in a network. Without any optimization, the space requirement is O(N), where N is the number of nodes. But with the explosive growth of the Internet, N is increasing at a steep rate, and the space requirement of the routing table is a matter of concern. This leads to the question: Can we reduce the size of the routing table? Given the value of N, what is the smallest amount of information that each individual node must store in their routing tables, so that every message eventually reaches its final destination? Our third example visits the problem of broadcasting in a network whose topology is represented by a connected graph.
    [Show full text]
  • Graph Traversal
    CSE 373: Graph traversal Michael Lee Friday, Feb 16, 2018 1 Solution: This algorithm is known as the 2-color algorithm. We can solve it by using any graph traversal algorithm, and alternating colors as we go from node to node. Warmup Warmup Given a graph, assign each node one of two colors such that no two adjacent vertices have the same color. (If it’s impossible to color the graph this way, your algorithm should say so). 2 Warmup Warmup Given a graph, assign each node one of two colors such that no two adjacent vertices have the same color. (If it’s impossible to color the graph this way, your algorithm should say so). Solution: This algorithm is known as the 2-color algorithm. We can solve it by using any graph traversal algorithm, and alternating colors as we go from node to node. 2 Problem: What if we want to traverse graphs following the edges? For example, can we... I Traverse a graph to find if there’s a connection from one node to another? I Determine if we can start from our node and touch every other node? I Find the shortest path between two nodes? Solution: Use graph traversal algorithms like breadth-first search and depth-first search Goal: How do we traverse graphs? Today’s goal: how do we traverse graphs? Idea 1: Just get a list of the vertices and loop over them 3 I Determine if we can start from our node and touch every other node? I Find the shortest path between two nodes? Solution: Use graph traversal algorithms like breadth-first search and depth-first search Goal: How do we traverse graphs? Today’s goal: how do we traverse graphs? Idea 1: Just get a list of the vertices and loop over them Problem: What if we want to traverse graphs following the edges? For example, can we..
    [Show full text]
  • Chapter 9 Graph Search
    Chapter 9 Graph Search The term graph search or graph traversal refers to a class of algorithms based on systematically visiting the vertices of a graph that can be used to compute various properties of graphs. In this chapter, we will introduce the concept of a graph search, describe a generalized algorithm for it, and describe a particular specialization, called priority-first search, that still remains relatively general. In the next several chapters, we will consider further specializations of the general graph-search algorithm, specifically the breadth-first-search and the depth-first-search algorithms, each of which is particularly useful in computing certain properties of graphs. To motivate graph search, let’s first consider the kinds of properties of a graph that we might be interested in computing. We might want to determine if one vertex is reachable from another. Recall that a vertex u is reachable from v if there is a (directed) path from v to u. We might also want to determine if an undirected graph is connected, or a directed graph is strongly connected—i.e. there is a path from every vertex to every other vertex. Or we might want to determine the shortest path from a vertex to another vertex. These properties all involve paths, so it makes sense to think about algorithms that follow paths. This is effectively the goal of graph-search. The idea of Graph search or graph traversal is to systematically visit (explore) the vertices in the graph by following paths. These searches typically start from a single vertex, although they can be generalized to start from a set of vertices.
    [Show full text]
  • Basic Graph Algorithms
    Basic Graph Algorithms Jaehyun Park CS 97SI Stanford University June 29, 2015 Outline Graphs Adjacency Matrix and Adjacency List Special Graphs Depth-First and Breadth-First Search Topological Sort Eulerian Circuit Minimum Spanning Tree (MST) Strongly Connected Components (SCC) Graphs 2 Graphs ◮ An abstract way of representing connectivity using nodes (also called vertices) and edges ◮ We will label the nodes from 1 to n ◮ m edges connect some pairs of nodes – Edges can be either one-directional (directed) or bidirectional ◮ Nodes and edges can have some auxiliary information Graphs 3 Why Study Graphs? ◮ Lots of problems formulated and solved in terms of graphs – Shortest path problems – Network flow problems – Matching problems – 2-SAT problem – Graph coloring problem – Traveling Salesman Problem (TSP): still unsolved! – and many more... Graphs 4 Outline Graphs Adjacency Matrix and Adjacency List Special Graphs Depth-First and Breadth-First Search Topological Sort Eulerian Circuit Minimum Spanning Tree (MST) Strongly Connected Components (SCC) Adjacency Matrix and Adjacency List 5 Storing Graphs ◮ Need to store both the set of nodes V and the set of edges E – Nodes can be stored in an array – Edges must be stored in some other way ◮ Want to support operations such as: – Retrieving all edges incident to a particular node – Testing if given two nodes are directly connected ◮ Use either adjacency matrix or adjacency list to store the edges Adjacency Matrix and Adjacency List 6 Adjacency Matrix ◮ An easy way to store connectivity information
    [Show full text]
  • B. Tech) (Four Year Regular Programme) (Applicable for Batches Admitted from 2018
    Academic Regulations Programme Structure & Detailed Syllabus Bachelor of Technology (B. Tech) (Four Year Regular Programme) (Applicable for Batches admitted from 2018) Department of Information Technology GOKARAJU RANGARAJU INSTITUTE OF ENGINEERING & TECHNOLOGY Bachupally, Kukatpally, Hyderabad, Telangana, India 500 090 Academic Regulations GOKARAJU RANGARAJU INSTITUTE OF ENGINEERING AND TECHNOLOGY, HYDERABAD DEPARTMENT OF INFORMATION TECHNOLOGY (B. Tech) GR18 REGULATIONS Gokaraju Rangaraju Institute of Engineering and Technology 2018 Regulations (GR18 Regulations) are given hereunder. These regulations govern the programmes offered by the Department of Information Technology with effect from the students admitted to the programmes in 2018-19 academic year. 1. Programme Offered: The programme offered by the Department is B. Tech in Information Technology, a four-year regular programme. 2. Medium of Instruction: The medium of instruction (including examinations and reports) is English. 3. Admissions: Admission to the B. Tech in Information Technology Programme shall be made subject to the eligibility, qualifications and specialization prescribed by the State Government/University from time to time. Admissions shall be made either on the basis of the merit rank obtained by the student in the common entrance examination conducted by the Government/Universityor on the basis of any other order of merit approved by the Government/University, subject to reservations as prescribed by the Government/University from time to time. 4. Programme Pattern: a) Each Academic year of study is divided in to two semesters. b) Minimum number of instruction days in each semester is 90. c) Grade points, based on percentage of marks awarded for each course will form the basis for calculation of SGPA (Semester Grade Point Average) and CGPA (Cumulative Grade Point Average).
    [Show full text]