```001// Exercise 4.3.21 4.3.22 (Solution published at http://algs4.cs.princeton.edu/)
002package algs43;
003import stdlib.*;
004import algs13.Queue;
005import algs15.WeightedUF;
006import algs24.IndexMinPQ;
007/* ****************************************************************************
008 *  Compilation:  javac PrimMST.java
009 *  Execution:    java PrimMST filename.txt
010 *  Dependencies: EdgeWeightedGraph.java Edge.java Queue.java
011 *                IndexMinPQ.java UF.java In.java StdOut.java
012 *  Data files:   http://algs4.cs.princeton.edu/43mst/tinyEWG.txt
013 *                http://algs4.cs.princeton.edu/43mst/mediumEWG.txt
014 *                http://algs4.cs.princeton.edu/43mst/largeEWG.txt
015 *
016 *  Compute a minimum spanning forest using Prim's algorithm.
017 *
018 *  %  java PrimMST tinyEWG.txt
019 *  1-7 0.19000
020 *  0-2 0.26000
021 *  2-3 0.17000
022 *  4-5 0.35000
023 *  5-7 0.28000
024 *  6-2 0.40000
025 *  0-7 0.16000
026 *  1.81000
027 *
028 *  % java PrimMST mediumEWG.txt
029 *  1-72   0.06506
030 *  2-86   0.05980
031 *  3-67   0.09725
032 *  4-55   0.06425
033 *  5-102  0.03834
034 *  6-129  0.05363
035 *  7-157  0.00516
036 *  ...
037 *  10.46351
038 *
039 *  % java PrimMST largeEWG.txt
040 *  ...
041 *  647.66307
042 *
043 ******************************************************************************/
044
045public class PrimMST {
046        private final Edge[] edgeTo;        // edgeTo[v] = shortest edge from tree vertex to non-tree vertex
047        private final double[] distTo;      // distTo[v] = weight of shortest such edge
048        private final boolean[] marked;     // marked[v] = true if v on tree, false otherwise
049        private final IndexMinPQ<Double> pq;
050
051        public PrimMST(EdgeWeightedGraph G) {
052                edgeTo = new Edge[G.V()];
053                distTo = new double[G.V()];
054                marked = new boolean[G.V()];
055                pq = new IndexMinPQ<>(G.V());
056                for (int v = 0; v < G.V(); v++) distTo[v] = Double.POSITIVE_INFINITY;
057
058                for (int v = 0; v < G.V(); v++)      // run from each vertex to find
059                        if (!marked[v]) prim(G, v);      // minimum spanning forest
060
061                // check optimality conditions
062                assert check(G);
063        }
064
065        // run Prim's algorithm in graph G, starting from vertex s
066        private void prim(EdgeWeightedGraph G, int s) {
067                distTo[s] = 0.0;
068                pq.insert(s, distTo[s]);
069                while (!pq.isEmpty()) {
070                        int v = pq.delMin();
071                        scan(G, v);
072                }
073        }
074
075        // scan vertex v
076        private void scan(EdgeWeightedGraph G, int v) {
077                marked[v] = true;
078                for (Edge e : G.adj(v)) {
079                        int w = e.other(v);
080                        if (marked[w]) continue;         // v-w is obsolete edge
081                        if (e.weight() < distTo[w]) {
082                                distTo[w] = e.weight();
083                                edgeTo[w] = e;
084                                if (pq.contains(w)) pq.decreaseKey(w, distTo[w]);
085                                else                pq.insert(w, distTo[w]);
086                        }
087                }
088        }
089
090        // return iterator of edges in MST
091        public Iterable<Edge> edges() {
092                Queue<Edge> mst = new Queue<>();
093                for (Edge e : edgeTo) {
094                        if (e != null) {
095                                mst.enqueue(e);
096                        }
097                }
098                return mst;
099        }
100
101
102        // return weight of MST
103        public double weight() {
104                double weight = 0.0;
105                for (Edge e : edges())
106                        weight += e.weight();
107                return weight;
108        }
109
110
111        // check optimality conditions (takes time proportional to E V lg* V)
112        private boolean check(EdgeWeightedGraph G) {
113
114                // check weight
115                double totalWeight = 0.0;
116                for (Edge e : edges()) {
117                        totalWeight += e.weight();
118                }
119                double EPSILON = 1E-12;
120                if (Math.abs(totalWeight - weight()) > EPSILON) {
121                        System.err.format("Weight of edges does not equal weight(): %f vs. %f\n", totalWeight, weight());
122                        return false;
123                }
124
125                // check that it is acyclic
126                WeightedUF uf = new WeightedUF(G.V());
127                for (Edge e : edges()) {
128                        int v = e.either(), w = e.other(v);
129                        if (uf.connected(v, w)) {
130                                System.err.println("Not a forest");
131                                return false;
132                        }
133                        uf.union(v, w);
134                }
135
136                // check that it is a spanning forest
137                for (Edge e : edges()) {
138                        int v = e.either(), w = e.other(v);
139                        if (!uf.connected(v, w)) {
140                                System.err.println("Not a spanning forest");
141                                return false;
142                        }
143                }
144
145                // check that it is a minimal spanning forest (cut optimality conditions)
146                for (Edge e : edges()) {
147                        int v = e.either(), w = e.other(v);
148
149                        // all edges in MST except e
150                        uf = new WeightedUF(G.V());
151                        for (Edge f : edges()) {
152                                int x = f.either(), y = f.other(x);
153                                if (f != e) uf.union(x, y);
154                        }
155
156                        // check that e is min weight edge in crossing cut
157                        for (Edge f : G.edges()) {
158                                int x = f.either(), y = f.other(x);
159                                if (!uf.connected(x, y)) {
160                                        if (f.weight() < e.weight()) {
161                                                System.err.println("Edge " + f + " violates cut optimality conditions");
162                                                return false;
163                                        }
164                                }
165                        }
166
167                }
168
169                return true;
170        }
171
172
173        public static void main(String[] args) {
174                args = new String[] { "data/10000EWG.txt" };
175                //args = new String[] { "data/mediumEWG.txt" };
176                //args = new String[] { "data/largeEWG.txt" };
177                In in = new In(args[0]);
178                EdgeWeightedGraph G = new EdgeWeightedGraph(in);
179                PrimMST mst = new PrimMST(G);
180                for (Edge e : mst.edges()) {
181                        StdOut.println(e);
182                }
183                StdOut.format("%.5f\n", mst.weight());
184        }
185
186
187}

```