Section 6.3: Shortest Path and Minimal Spanning Tree

Abstract:

Several algorithms are described for tracing the shortest path for a simple, positively weighted, connected graph. In addition, algorithms for finding a minimal spanning tree are also described, which are useful for trimming a graph to a subgraph which leaves all nodes connected, but eliminates ``unnecessary'' connections.

Shortest Path Algorithms

Last class I asked you to consider your own intuitive schemes for finding the shortest path between two nodes. Some suggestions included

• Examining all paths (exhaustive) and choosing the simplest (you would need to check that no arcs are used twice);
• Recursion.

We noted that, if there are n nodes, you need at most a path of length n-1.

It is relatively easy to come up with algorithms to solve this problem, but of course some ways are better than others. We'll look at a couple of standard algorithms for carrying out this task.

Dijkstra's Algorithm

This algorithm was first described by Edsger W. Dijkstra. Here is a web-based example of the workings of the algorithm (where there is no stated destination node).

• Given two nodes x and y in a simple, connected, positively weighted graph. We seek the shortest path from x to y (assume a non-directed graph).
• Represent the graph by its adjacency matrix (with distances between non-adjacent nodes set to ).
• A settled node is one whose distance from x is known. Initially, the set of settled nodes is only .
• We grow IN by adding in the next nearest node to x via those already settled. When y falls into IN, we're done.

We keep track of two arrays, indexed by the nodes of graph G:

• an array d indexed by the nodes z, of distances of z to x; and
• an array s indexed by the nodes z, of the node adjacent to a given node z on the shortest path from z to x (so far).

When y enters IN, we can use s to trace the shortest path.

Let's look at an example:

Exercise #3, p. 437

Other shortest path algorithms

The Bellman-Ford Algorithm (AnotherShortestPath, p. 438) operates in a fashion similar to Dijkstra's algorithm, only it finds the shortest distance from x to every other node as described in the book (one could add a termination step, of course).

Each node keeps an eye on its adjacent nodes:

• if they don't change, no change in node
• if they change, reevaluate based on the weight of that arc.

Exercise #10, p. 439

Floyd's algorithm (the algorithm AllPairsShortestPath, p. 439) is simpler, stupider, but has the advantage that it produces the shortest distance between any two nodes in the graph. Sometimes this is desired, rather than the distance between any special pair. It too works with the adjacency matrix representation of the graph (modified to contain off the diagonal).

It simply uses brute force to compare direct paths between a pair and indirect paths between the same pair: we compare

to A(i,j), to see if it's shorter to go from i to j via k.

Exercise #13 (homework problem), p. 439.

Minimal Spanning Trees

Definition: A spanning tree for a connected graph G is a non-rooted tree containing the nodes of the graph and a subset of the arcs of G. A minimal spanning tree is a spanning tree of least weight of a simple, weighted, connected graph G.

Prim's algorithm is a simple one for constructing a minimal spanning tree (these may not be unique!):

• Pick a node at random (they all must figure in the spanning tree, and it doesn't matter where you start). This node is the start of your tree T.
• Follow the arc from the tree T to the nearest adjacent node, and incorporate that arc and node into T (there may be ties - pick one).
• Iterate!

Exercise #18, p. 440.

Kruskal's algorithm is an alternative method for generating a minimal spanning tree. It works by building up a spanning tree from the arcs, ordered from smallest in weight to largest:

Exercise #21, p. 441.

...

Dijkstra, whom we met earlier in the course, was the one responsible for the quote that ``the quality of programmers is a decreasing function of the density of GO TO statements in the programs they produce.'' (from a letter to the editor of Communications of the ACM, circa 1968)