// check optimality conditions (takes time proportional to E V lg* V)
private boolean check(EdgeWeightedGraph G) {
// check weight
double totalWeight = 0.0;
for (Edge e : edges()) {
totalWeight += e.weight();
}
if (Math.abs(totalWeight - weight()) > FLOATING_POINT_EPSILON) {
System.err.printf("Weight of edges does not equal weight(): %f vs. %f\n", totalWeight, weight());
return false;
}
// check that it is acyclic
UF uf = new UF(G.V());
for (Edge e : edges()) {
int v = e.either(), w = e.other(v);
if (uf.connected(v, w)) {
System.err.println("Not a forest");
return false;
}
uf.union(v, w);
}
// check that it is a spanning forest
for (Edge e : G.edges()) {
int v = e.either(), w = e.other(v);
if (!uf.connected(v, w)) {
System.err.println("Not a spanning forest");
return false;
}
}
// check that it is a minimal spanning forest (cut optimality conditions)
for (Edge e : edges()) {
// all edges in MST except e
uf = new UF(G.V());
for (Edge f : edges()) {
int x = f.either(), y = f.other(x);
if (f != e) uf.union(x, y);
}
// check that e is min weight edge in crossing cut
for (Edge f : G.edges()) {
int x = f.either(), y = f.other(x);
if (!uf.connected(x, y)) {
if (f.weight() < e.weight()) {
System.err.println("Edge " + f + " violates cut optimality conditions");
return false;
}
}
}
}
return true;
}
private double weight; // weight of MST
/**
* Compute a minimum spanning tree (or forest) of an edge-weighted graph.
* @param G the edge-weighted graph
*/
public BoruvkaMST(EdgeWeightedGraph G) {
UF uf = new UF(G.V());
// repeat at most log V times or until we have V-1 edges
for (int t = 1; t < G.V() && mst.size() < G.V() - 1; t = t + t) {
// foreach tree in forest, find closest edge
// if edge weights are equal, ties are broken in favor of first edge in G.edges()
Edge[] closest = new Edge[G.V()];
for (Edge e : G.edges()) {
int v = e.either(), w = e.other(v);
int i = uf.find(v), j = uf.find(w);
if (i == j) continue; // same tree
if (closest[i] == null || less(e, closest[i])) closest[i] = e;
if (closest[j] == null || less(e, closest[j])) closest[j] = e;
}
// add newly discovered edges to MST
for (int i = 0; i < G.V(); i++) {
Edge e = closest[i];
if (e != null) {
int v = e.either(), w = e.other(v);
// don't add the same edge twice
if (!uf.connected(v, w)) {
mst.add(e);
weight += e.weight();
uf.union(v, w);
}
}
}
}
// check optimality conditions
assert check(G);
}
private IndexMinPQ<Double> pq; // priority queue of vertices
/**
* Computes a shortest-paths tree from the source vertex {@code s} to every
* other vertex in the edge-weighted graph {@code G}.
*
* @param G the edge-weighted digraph
* @param s the source vertex
* @throws IllegalArgumentException if an edge weight is negative
* @throws IllegalArgumentException unless {@code 0 <= s < V}
*/
public DijkstraUndirectedSP(EdgeWeightedGraph G, int s) {
for (Edge e : G.edges()) {
if (e.weight() < 0)
throw new IllegalArgumentException("edge " + e + " has negative weight");
}
distTo = new double[G.V()];
edgeTo = new Edge[G.V()];
validateVertex(s);
for (int v = 0; v < G.V(); v++)
distTo[v] = Double.POSITIVE_INFINITY;
distTo[s] = 0.0;
// relax vertices in order of distance from s
pq = new IndexMinPQ<Double>(G.V());
pq.insert(s, distTo[s]);
while (!pq.isEmpty()) {
int v = pq.delMin();
for (Edge e : G.adj(v))
relax(e, v);
}
// check optimality conditions
assert check(G, s);
}
// check optimality conditions:
// (i) for all edges e = v-w: distTo[w] <= distTo[v] + e.weight()
// (ii) for all edge e = v-w on the SPT: distTo[w] == distTo[v] + e.weight()
private boolean check(EdgeWeightedGraph G, int s) {
// check that edge weights are nonnegative
for (Edge e : G.edges()) {
if (e.weight() < 0) {
System.err.println("negative edge weight detected");
return false;
}
}
// check that distTo[v] and edgeTo[v] are consistent
if (distTo[s] != 0.0 || edgeTo[s] != null) {
System.err.println("distTo[s] and edgeTo[s] inconsistent");
return false;
}
for (int v = 0; v < G.V(); v++) {
if (v == s) continue;
if (edgeTo[v] == null && distTo[v] != Double.POSITIVE_INFINITY) {
System.err.println("distTo[] and edgeTo[] inconsistent");
return false;
}
}
// check that all edges e = v-w satisfy distTo[w] <= distTo[v] + e.weight()
for (int v = 0; v < G.V(); v++) {
for (Edge e : G.adj(v)) {
int w = e.other(v);
if (distTo[v] + e.weight() < distTo[w]) {
System.err.println("edge " + e + " not relaxed");
return false;
}
}
}
// check that all edges e = v-w on SPT satisfy distTo[w] == distTo[v] + e.weight()
for (int w = 0; w < G.V(); w++) {
if (edgeTo[w] == null) continue;
Edge e = edgeTo[w];
if (w != e.either() && w != e.other(e.either())) return false;
int v = e.other(w);
if (distTo[v] + e.weight() != distTo[w]) {
System.err.println("edge " + e + " on shortest path not tight");
return false;
}
}
return true;
}
/**
* Checks optimality conditions.
*
* @param G the edge-weighted graph
* @return {@code true} if optimality conditions are fine
*/
private boolean check(EdgeWeightedGraph G) {
// compute min st-cut for all pairs s and t
// shortcut: s must appear on one side of global mincut,
// so it suffices to try all pairs s-v for some fixed s
double value = Double.POSITIVE_INFINITY;
for (int s = 0, t = 1; t < G.V(); t++) {
FlowNetwork F = new FlowNetwork(G.V());
for (Edge e : G.edges()) {
int v = e.either(), w = e.other(v);
F.addEdge(new FlowEdge(v, w, e.weight()));
F.addEdge(new FlowEdge(w, v, e.weight()));
}
FordFulkerson maxflow = new FordFulkerson(F, s, t);
value = Math.min(value, maxflow.value());
}
if (Math.abs(weight - value) > FLOATING_POINT_EPSILON) {
System.err.println("Min cut weight = " + weight + " , max flow value = " + value);
return false;
}
return true;
}
private Queue<Edge> mst = new Queue<Edge>(); // edges in MST
/**
* Compute a minimum spanning tree (or forest) of an edge-weighted graph.
* @param G the edge-weighted graph
*/
public KruskalMST(EdgeWeightedGraph G) {
// more efficient to build heap by passing array of edges
MinPQ<Edge> pq = new MinPQ<Edge>();
for (Edge e : G.edges()) {
pq.insert(e);
}
// run greedy algorithm
UF uf = new UF(G.V());
while (!pq.isEmpty() && mst.size() < G.V() - 1) {
Edge e = pq.delMin();
int v = e.either();
int w = e.other(v);
if (!uf.connected(v, w)) { // v-w does not create a cycle
uf.union(v, w); // merge v and w components
mst.enqueue(e); // add edge e to mst
weight += e.weight();
}
}
// check optimality conditions
assert check(G);
}
/**
* Validates the edge-weighted graph.
*
* @param G the edge-weighted graph
* @throws IllegalArgumentException if the number of vertices of {@code G}
* is less than {@code 2} or if any edge weight is negative
*/
private void validate(EdgeWeightedGraph G) {
if (G.V() < 2) throw new IllegalArgumentException("number of vertices of G is less than 2");
for (Edge e : G.edges()) {
if (e.weight() < 0) throw new IllegalArgumentException("edge " + e + " has negative weight");
}
}