Weighted Graph Algorithms Weighted shortest path problem Dijkstras - - PowerPoint PPT Presentation

Weighted Graph Algorithms

• Weighted shortest path problem

– Dijkstra’s algorithm (single‐source, non‐negative edge weight)

• All pairs shortest path

– Warshall’s algorithm

Definition of a path

• Consider a directed graph G=(V, E) with edge‐

weight function w: ER.

– Weight of edge v1 v2: w(v1 ,v2 )

• The weight of a path p=v1

v2 … vk is defined to be w(p)= sum w(vi , vi+1 ), i=1..k‐1

Shortest path

• A shortest path from u to v is a path of

minimum weight from u to v

– There might be multiple paths from u to v – The shortest path weight from u to v is defined as –

Single source shortest path

• Problem: from a given source vertex s, find the

shortest path weights from s to all vertices in V.

– If all edge weights w(u,v) are non‐negative, all shortest path weights must exist

• Idea: greedy

– Maintain a set S of vertices whose shortest path weights from s are known. – At each step, add to S the vertex v, whose distance estimate from s is minimal – Update the distance estimates of all vertices adjacent to v.

Dijkstra’s algorithm

Running time analysis

Running time analysis

• Unweighted

shortest path problem

• Dijkstra

with negative weights