CS 3343: Analysis of Algorithms

CS 3343: Analysis of Algorithms Lecture 21: Introduction to Graphs

0 0 Uniform-profit Restaurant location problem Goal: maximize number of restaurants open Subject to: distance constraint (min-separation >= 10) 5 2 2 6 6 3 6 10 7 6 9 15 7 d 5 7 9 15 10 0 0

Events scheduling problem Goal: maximize number of non-conflicting events e6 e8 e3 e7 e4 e5 e9 e1 e2 Time

item Weight (LB) Value ($) $ / LB 1 2 2 1 2 4 3 0.75 3 3 3 1 4 5 6 1.2 5 2 4 2 6 6 9 1.5 Fractional knapsack problem • Goal: maximize value without exceeding bag capacity • Weight limit: 10LB

Example • Goal: maximize value without exceeding bag capacity • Weight limit: 10LB • 2 + 6 + 2 = 10 LB • 4 + 9 + 1.2*2 = 15.4

The remaining lectures • Graph algorithms • Very important in practice • Tons of computational problems can be defined in terms of graphs • We’ll study a few interesting ones • Minimum spanning tree • Shortest path • Graph search • Topological sort, connected components

Graphs • A graph G = (V, E) • V = set of vertices • E = set of edges = subset of V  V • Thus |E| = O(|V|2) 1 Vertices: {1, 2, 3, 4} Edges: {(1, 2), (2, 3), (1, 3), (4, 3)} 2 4 3

Graph Variations (1) • Directed / undirected: • In an undirected graph: • Edge (u,v)  E implies edge (v,u)  E • Road networks between cities • In a directed graph: • Edge (u,v): uv does not imply vu • Street networks in downtown • Degree of vertex v: • The number of edges adjacency to v • For directed graph, there are in-degree and out-degree

1 1 2 4 2 4 3 3 In-degree = 3 Out-degree = 0 Degree = 3 Directed Undirected

Graph Variations (2) • Weighted / unweighted: • In a weighted graph, each edge or vertex has an associated weight (numerical value) • E.g., a road map: edges might be weighted w/ distance 1 1 0.3 2 4 2 4 1.2 0.4 1.9 3 3 Weighted Unweighted

Graph Variations (3) • Connected / disconnected: • A connected graphhas a path from every vertex to every other • A directed graph is strongly connectedif there is a directed path between any two vertices 1 2 4 Connected but not strongly connected 3

Graph Variations (4) • Dense / sparse: • Graphs are sparsewhen the number of edges is linear to the number of vertices • |E|  O(|V|) • Graphs are densewhen the number of edges is quadratic to the number of vertices • |E|  O(|V|2) • Most graphs of interest are sparse • If you know you are dealing with dense or sparse graphs, different data structures may make sense

Representing Graphs • Assume V = {1, 2, …, n} • An adjacency matrixrepresents the graph as a n x n matrix A: • A[i, j] = 1 if edge (i, j)  E = 0 if edge (i, j)  E • For weighted graph • A[i, j] = wij if edge (i, j)  E = 0 if edge (i, j)  E • For undirected graph • Matrix is symmetric: A[i, j] = A[j, i]

Graphs: Adjacency Matrix • Example: 1 2 4 3

Graphs: Adjacency Matrix • Example: 1 2 4 3 How much storage does the adjacency matrix require? A: O(V2)

0 1 1 0 1 0 1 0 1 1 0 1 0 0 1 0 Graphs: Adjacency Matrix • Example: A 1 2 3 4 1 1 2 2 4 3 3 4 Undirected graph

0 5 6 0 5 0 9 0 6 9 0 4 0 0 4 0 Graphs: Adjacency Matrix • Example: A 1 2 3 4 1 1 5 2 6 2 4 3 9 4 3 4 Weighted graph

Graphs: Adjacency Matrix • Time to answer if there is an edge between vertex u and v: Θ(1) • Memory required: Θ(n2) regardless of |E| • Usually too much storage for large graphs • But can be very efficient for small graphs • Most large interesting graphs are sparse • E.g., road networks (due to limit on junctions) • For this reason the adjacency list is often a more appropriate representation

Graphs: Adjacency List • Adjacency list: for each vertex v  V, store a list of vertices adjacent to v • Example: • Adj[1] = {2,3} • Adj[2] = {3} • Adj[3] = {} • Adj[4] = {3} • Variation: can also keep a list of edges coming into vertex 1 2 4 3

Graph representations • Adjacency list 1 2 3 3 2 4 3 3 How much storage does the adjacency list require? A: O(V+E)

A 1 2 3 4 1 0 1 1 0 2 1 0 1 0 3 1 1 0 1 4 0 0 1 0 Graph representations • Undirected graph 1 2 4 3 2 3 1 3 1 2 4 3

A 1 2 3 4 1 0 5 6 0 2 5 0 9 0 3 6 9 0 4 4 0 0 4 0 Graph representations • Weighted graph 1 5 6 2 4 9 4 3 2,5 3,6 1,5 3,9 1,6 2,9 4,4 3,4

Tradeoffs between the two representations |V| = n, |E| = m Both representations are very useful and have different properties.

6 4 5 9 14 2 10 15 3 8 Minimum Spanning Tree • Problem: given a connected, undirected, weighted graph:

Minimum Spanning Tree • Problem: given a connected, undirected, weighted graph, find a spanning tree using edges that minimize the total weight 6 4 5 9 • A spanning tree is a tree that connects all vertices • Number of edges = ? • A spanning tree has no designated root. 14 2 10 15 3 8

How to find MST? • Connect every node to the closest node? • Does not guarantee a spanning tree

T2 T1 T1’ v u Minimum Spanning Tree • MSTs satisfy the optimal substructure property: an optimal tree is composed of optimal subtrees • Let T be an MST of G with an edge (u,v) in the middle • Removing (u,v) partitions T into two trees T1 and T2 • w(T) = w(u,v) + w(T1) + w(T2) • Claim 1:T1 is an MST of G1 = (V1, E1), and T2 is an MST of G2 = (V2, E2) • Proof by contradiction: • if T1 is not optimal, we can replace T1 with a better spanning tree, T1’ • T1’, T2 and (u, v) form a new spanning tree T’ • W(T’) < W(T). Contradiction.

y x Minimum Spanning Tree • MSTs satisfy the optimal substructure property: an optimal tree is composed of optimal subtrees • Let T be an MST of G with an edge (u,v) in the middle • Removing (u,v) partitions T into two trees T1 and T2 • w(T) = w(u,v) + w(T1) + w(T2) • Claim 2:(u, v) is the lightest edge connecting G1 = (V1, E1) and G2 = (V2, E2) • Proof by contradiction: • if (u, v) is not the lightest edge, we can remove it, and reconnect T1 and T2 with a lighter edge (x, y) • T1, T2 and (x, y) form a new spanning tree T’ • W(T’) < W(T). Contradiction. T2 T1 v u

Algorithms • Generic idea: • Compute MSTs for sub-graphs • Connect two MSTs for sub-graphs with the lightest edge • Two of the most well-known algorithms • Prim’s algorithm • Kruskal’s algorithm • Let’s first talk about the ideas behind the algorithms without worrying about the implementation and analysis

Prim’s algorithm • Basic idea: • Start from an arbitrary single node • A MST for a single node has no edge • Gradually build up a single larger and larger MST 6 5 Not yet discovered 7 Fully explored nodes Discovered but not fully explored nodes

Prim’s algorithm • Basic idea: • Start from an arbitrary single node • A MST for a single node has no edge • Gradually build up a single larger and larger MST 2 6 5 9 Not yet discovered 4 7 Fully explored nodes Discovered but not fully explored nodes

Prim’s algorithm • Basic idea: • Start from an arbitrary single node • A MST for a single node has no edge • Gradually build up a single larger and larger MST 2 6 5 9 4 7

Prim’s algorithm in words • Randomly pick a vertex as the initial tree T • Gradually expand into a MST: • For each vertex that is not in T but directly connected to some nodes in T • Compute its minimum distance to any vertex in T • Select the vertex that is closest to T • Add it to T

Example a 6 12 9 5 b f g 7 14 15 8 c e h 10 3 d

Example a 6 12 9 5 b f g 7 14 15 8 c e h 10 3 d Total weight = 3 + 8 + 6 + 5 + 7 + 9 + 15 = 53

Kruskal’s algorithm • Basic idea: • Grow many small trees • Find two trees that are closest (i.e., connected with the lightest edge), join them with the lightest edge • Terminate when a single tree forms

Claim • If edge (u, v) is the lightest among all edges, (u, v) is in a MST • Proof by contradiction: • Suppose that (u, v) is not in any MST • Given a MST T, if we connect (u, v), we create a cycle • Remove an edge in the cycle, have a new tree T’ • W(T’) < W(T) By the same argument, the second, third, …, lightest edges, if they do not create a cycle, must be in MST v u

Kruskal’s algorithm in words • Procedure: • Sort all edges into non-decreasing order • Initially each node is in its own tree • For each edge in the sorted list • If the edge connects two separate trees, then • join the two trees together with that edge

Example c-d: 3 b-f: 5 b-a: 6 f-e: 7 b-d: 8 f-g: 9 d-e: 10 a-f: 12 b-c: 14 e-h: 15 a 6 12 9 5 b f g 7 14 15 8 c e h 10 3 d

CS 3343: Analysis of Algorithms