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chapter Single-Source Shortest Paths Problem Definition Shortest paths and Relaxation Dijkstra’s algorithm (can be viewed as a greedy algorithm)

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chapter252 Problem Definition: Real problem: A motorist wishes to find the shortest possible route from Chicago to Boston.Given a road map of the United States on which the distance between each pair of adjacent intersections is marked, how can we determine this shortest route? Formal definition: Given a directed graph G=(V, E, W), where each edge has a weight, find a shortest path from s to v for some interesting vertices s and v. s—source v—destination.

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chapter253 Find a shortest path from station A to station B. -need serious thinking to get a correct algorithm. A B

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chapter254 Shortest path: The weight of path p= is the sum of the weights of its constituent edges: The cost of the shortest path from s to v is denoted as (s, v).

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chapter255 Negative-Weight edges: Edge weight may be negative. negative-weight cycles– the total weight in the cycle (circuit) is negative. If no negative-weight cycles reachable from the source s, then for all v V, the shortest-path weight remains well defined,even if it has a negative value. If there is a negative-weight cycle on some path from s to v, we define = -.

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chapter256 Figure1 Negative edge weights in a directed graph.Shown within each vertex is its shortest-path weight from source s.Because vertices e and f form a negative-weight cycle reachable from s,they have shortest-path weights of -. Because vertex g is reachable from a vertex whose shortest path is -,it,too,has a shortest-path weight of -.Vertices such as h, i,and j are not reachable from s,and so their shortest-path weights are, even though they lie on a negative-weight cycle hi j s ab cd e f g

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chapter257 Representing shortest paths: we maintain for each vertex v V, a predecessor [ v] that is the vertex in the shortest path right before v. With the values of, a backtracking process can give the shortest path. (We will discuss that after the algorithm is given)

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chapter258 Observation: (basic) Suppose that a shortest path p from a source s to a vertex v can be decomposed into s u v for some vertex u and path p’. Then, the weight of a shortest path from s to v is We do not know what is u for v, but we know u is in V and we can try all nodes in V in O(n) time. Also, if u does not exist, the edge (s, v) is the shortest. Question: how to find (s, u), the first shortest from s to some node?

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chapter259 Relaxation: The process of relaxing an edge (u,v) consists of testing whether we can improve the shortest path to v found so far by going through u and,if so,updating d[v] and [v]. RELAX(u,v,w) if d[v]>d[u]+w(u,v) then d[v] d[u]+w(u,v) (based on observation) [v] u

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chapter2510 Figure2 Relaxation of an edge (u,v).The shortest-path estimate of each vertex is shown within the vertex. (a)Because d[v]>d[u]+w(u,v) prior to relaxation, the value of d[v] decreases. (b)Here, d[v] d[u]+w(u,v) before the relaxation step,so d[v] is unchanged by relaxation. 59 uv 57 uv 56 uv 56 uv (a) (b) RELAX(u,v)

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chapter2511 Initialization: For each vertex v V, d[v] denotes an upper bound on the weight of a shortest path from source s to v. d[v]– will be (s, v) after the execution of the algorithm. initialize d[v] and [v] as follows:. INITIALIZE-SINGLE-SOURCE(G,s) for each vertex v V[G] do d[v] [v] NIL d[s] 0

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chapter2512 Dijkstra’s Algorithm: Dijkstra’s algorithm assumes that w(e) 0 for each e in the graph. maintain a set S of vertices such that –Every vertex v S, d[v]= (s, v), i.e., the shortest-path from s to v has been found. (Intial values: S=empty, d[s]=0 and d[v]= ) (a) select the vertex u V-S such that d[u]=min {d[x]|x V-S}. Set S=S {u} (b) for each node v adjacent to u do RELAX(u, v, w). Repeat step (a) and (b) until S=V.

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chapter2513 Continue: DIJKSTRA(G,w,s): INITIALIZE-SINGLE-SOURCE(G,s) S Q V[G] while Q do u EXTRACT -MIN(Q) S S {u} for each vertex v Adj[u] do RELAX(u,v,w)

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chapter2514 Implementation: a priority queue Q stores vertices in V-S, keyed by their d[] values. the graph G is represented by adjacency lists.

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chapter s uv x y (a)

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chapter /s 10/s s uv x y 8 8 (b) (s,x) is the shortest path using one edge. It is also the shortest path from s to x.

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chapter /x 14/x 5/s 8/x s uv x y (c)

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chapter /x 13/y 5/s 8/x s uv x y (d)

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chapter /x 9/u 5/s 8/x s uv x y (e)

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chapter /x 9/u 5/s 8/x s uv x y (f) Backtracking: v-u-x-s

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chapter2521 Theorem: Consider the set S at any time in the algorithm’s execution. For each v S, the path P v is a shortest s-v path. Proof: We prove it by induction on |S|. 1.If |S|=1, then the theorem holds. (Because d[s]=0 and S={s}.) 2.Suppose that the theorem is true for |S|=k for some k>0. 3.Now, we grow S to size k+1 by adding the node v.

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chapter2522 Proof: (continue) Now, we grow S to size k+1 by adding the node v. Let (u, v) be the last edge on our s-v path P v, i.e., d[v]=d[u]+w(u, v). Consider any other path from P: s,…,x,y, …, v. (red in the Fig.) Case 1: y is the first node that is not in S and x S. Since we always select the node with the smallest value d[] in the algorithm, we have d[v] d[y]. Moreover, the length of each edge is 0. Thus, the length of P d[y] d[v]. That is, the length of any path d[v]. s x y uv Set S Case 2: such a y does not exist. d[v]=d[u]+w(u, v) d[x]+w(x, v). That is, the length of any path d[v].

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chapter2523 The algorithm does not work if there are negative weight edges in the graph s v u S->v is shorter than s->u, but it is longer than s->u->v.

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chapter2524 Time complexity of Dijkstra’s Algorithm: Time complexity depends on implementation of the Queue. Method 1: Use an array to story the Queue EXTRACT -MIN(Q) --takes O(|V|) time. –Totally, there are |V| EXTRACT -MIN(Q)’s. – time for |V| EXTRACT -MIN(Q)’s is O(|V| 2 ). RELAX(u,v,w) --takes O(1) time. –Totally |E| RELAX(u, v, w)’s are required. – time for |E| RELAX(u,v,w)’s is O(|E|). Total time required is O(|V| 2 +|E|)=O(|V| 2 ) Backtracking with [] gives the shortest path in inverse order. Method 2: The priority queue is implemented as a adaptable heap. It takes O(log n) time to do EXTRACT- MIN(Q) as well as | RELAX(u,v,w). The total running time is O(|E|log |V| ).

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