--- a
+++ b/network/Graph.java
@@ -0,0 +1,241 @@
+package network;
+
+/******************************************************************************
+ *  Compilation:  javac Graph.java        
+ *  Execution:    java Graph input.txt
+ *  Dependencies: Bag.java Stack.java In.java StdOut.java
+ *  Data files:   https://algs4.cs.princeton.edu/41graph/tinyG.txt
+ *                https://algs4.cs.princeton.edu/41graph/mediumG.txt
+ *                https://algs4.cs.princeton.edu/41graph/largeG.txt
+ *
+ *  A graph, implemented using an array of sets.
+ *  Parallel edges and self-loops allowed.
+ *
+ *  % java Graph tinyG.txt
+ *  13 vertices, 13 edges 
+ *  0: 6 2 1 5 
+ *  1: 0 
+ *  2: 0 
+ *  3: 5 4 
+ *  4: 5 6 3 
+ *  5: 3 4 0 
+ *  6: 0 4 
+ *  7: 8 
+ *  8: 7 
+ *  9: 11 10 12 
+ *  10: 9 
+ *  11: 9 12 
+ *  12: 11 9 
+ *
+ *  % java Graph mediumG.txt
+ *  250 vertices, 1273 edges 
+ *  0: 225 222 211 209 204 202 191 176 163 160 149 114 97 80 68 59 58 49 44 24 15 
+ *  1: 220 203 200 194 189 164 150 130 107 72 
+ *  2: 141 110 108 86 79 51 42 18 14 
+ *  ...
+ *  
+ ******************************************************************************/
+
+import java.util.NoSuchElementException;
+import java.util.Stack;
+
+/**
+ *  The {@code Graph} class represents an undirected graph of vertices
+ *  named 0 through <em>V</em> – 1.
+ *  It supports the following two primary operations: add an edge to the graph,
+ *  iterate over all of the vertices adjacent to a vertex. It also provides
+ *  methods for returning the number of vertices <em>V</em> and the number
+ *  of edges <em>E</em>. Parallel edges and self-loops are permitted.
+ *  By convention, a self-loop <em>v</em>-<em>v</em> appears in the
+ *  adjacency list of <em>v</em> twice and contributes two to the degree
+ *  of <em>v</em>.
+ *  <p>
+ *  This implementation uses an adjacency-lists representation, which 
+ *  is a vertex-indexed array of {@link Bag} objects.
+ *  All operations take constant time (in the worst case) except
+ *  iterating over the vertices adjacent to a given vertex, which takes
+ *  time proportional to the number of such vertices.
+ *  <p>
+ *  For additional documentation, see <a href="https://algs4.cs.princeton.edu/41graph">Section 4.1</a>
+ *  of <i>Algorithms, 4th Edition</i> by Robert Sedgewick and Kevin Wayne.
+ *
+ *  @author Robert Sedgewick
+ *  @author Kevin Wayne
+ */
+public class Graph {
+    private static final String NEWLINE = System.getProperty("line.separator");
+
+    private final int V;
+    private int E;
+    private Bag<Integer>[] adj;
+    
+    /**
+     * Initializes an empty graph with {@code V} vertices and 0 edges.
+     * param V the number of vertices
+     *
+     * @param  V number of vertices
+     * @throws IllegalArgumentException if {@code V < 0}
+     */
+    public Graph(int V) {
+        if (V < 0) throw new IllegalArgumentException("Number of vertices must be nonnegative");
+        this.V = V;
+        this.E = 0;
+        adj = (Bag<Integer>[]) new Bag[V];
+        for (int v = 0; v < V; v++) {
+            adj[v] = new Bag<Integer>();
+        }
+    }
+
+    /**  
+     * Initializes a graph from the specified input stream.
+     * The format is the number of vertices <em>V</em>,
+     * followed by the number of edges <em>E</em>,
+     * followed by <em>E</em> pairs of vertices, with each entry separated by whitespace.
+     *
+     * @param  in the input stream
+     * @throws IllegalArgumentException if the endpoints of any edge are not in prescribed range
+     * @throws IllegalArgumentException if the number of vertices or edges is negative
+     * @throws IllegalArgumentException if the input stream is in the wrong format
+     */
+    public Graph(In in) {
+        try {
+            this.V = in.readInt();
+            if (V < 0) throw new IllegalArgumentException("number of vertices in a Graph must be nonnegative");
+            adj = (Bag<Integer>[]) new Bag[V];
+            for (int v = 0; v < V; v++) {
+                adj[v] = new Bag<Integer>();
+            }
+            int E = in.readInt();
+            if (E < 0) throw new IllegalArgumentException("number of edges in a Graph must be nonnegative");
+            for (int i = 0; i < E; i++) {
+                int v = in.readInt();
+                int w = in.readInt();
+                validateVertex(v);
+                validateVertex(w);
+                addEdge(v, w); 
+            }
+        }
+        catch (NoSuchElementException e) {
+            throw new IllegalArgumentException("invalid input format in Graph constructor", e);
+        }
+    }
+
+
+    /**
+     * Initializes a new graph that is a deep copy of {@code G}.
+     *
+     * @param  G the graph to copy
+     */
+    public Graph(Graph G) {
+        this(G.V());
+        this.E = G.E();
+        for (int v = 0; v < G.V(); v++) {
+            // reverse so that adjacency list is in same order as original
+            Stack<Integer> reverse = new Stack<Integer>();
+            for (int w : G.adj[v]) {
+                reverse.push(w);
+            }
+            for (int w : reverse) {
+                adj[v].add(w);
+            }
+        }
+    }
+
+    /**
+     * Returns the number of vertices in this graph.
+     *
+     * @return the number of vertices in this graph
+     */
+    public int V() {
+        return V;
+    }
+
+    /**
+     * Returns the number of edges in this graph.
+     *
+     * @return the number of edges in this graph
+     */
+    public int E() {
+        return E;
+    }
+
+    // throw an IllegalArgumentException unless {@code 0 <= v < V}
+    private void validateVertex(int v) {
+        if (v < 0 || v >= V)
+            throw new IllegalArgumentException("vertex " + v + " is not between 0 and " + (V-1));
+    }
+
+    /**
+     * Adds the undirected edge v-w to this graph.
+     *
+     * @param  v one vertex in the edge
+     * @param  w the other vertex in the edge
+     * @throws IllegalArgumentException unless both {@code 0 <= v < V} and {@code 0 <= w < V}
+     */
+    public void addEdge(int v, int w) {
+        validateVertex(v);
+        validateVertex(w);
+        E++;
+        adj[v].add(w);
+        adj[w].add(v);
+    }
+
+
+    /**
+     * Returns the vertices adjacent to vertex {@code v}.
+     *
+     * @param  v the vertex
+     * @return the vertices adjacent to vertex {@code v}, as an iterable
+     * @throws IllegalArgumentException unless {@code 0 <= v < V}
+     */
+    public Iterable<Integer> adj(int v) {
+        validateVertex(v);
+        return adj[v];
+    }
+
+    /**
+     * Returns the degree of vertex {@code v}.
+     *
+     * @param  v the vertex
+     * @return the degree of vertex {@code v}
+     * @throws IllegalArgumentException unless {@code 0 <= v < V}
+     */
+    public int degree(int v) {
+        validateVertex(v);
+        return adj[v].size();
+    }
+
+
+    /**
+     * Returns a string representation of this graph.
+     *
+     * @return the number of vertices <em>V</em>, followed by the number of edges <em>E</em>,
+     *         followed by the <em>V</em> adjacency lists
+     */
+    public String toString() {
+        StringBuilder s = new StringBuilder();
+        s.append(V + " vertices, " + E + " edges " + NEWLINE);
+        for (int v = 0; v < V; v++) {
+            s.append(v + ": ");
+            for (int w : adj[v]) {
+                s.append(w + " ");
+            }
+            s.append(NEWLINE);
+        }
+        return s.toString();
+    }
+
+
+    /**
+     * Unit tests the {@code Graph} data type.
+     *
+     * @param args the command-line arguments
+     */
+    public static void main(String[] args) {
+        In in = new In(args[0]);
+        Graph G = new Graph(in);
+        StdOut.println(G);
+    }
+
+}
+