Interview Preparation

Directed Graphs

Glossary

  • A self-loop is an edge that connects a vertex to itself.

  • Two edges are parallel if they connect the same ordered pair of vertices.

  • The outdegree of a vertex is the number of edges pointing from it.

  • The indegree of a vertex is the number of edges pointing to it.

  • A subgraph is a subset of a digraph's edges (and associated vertices) that constitutes a digraph.

  • A directed path in a digraph is a sequence of vertices in which there is a (directed) edge pointing from each vertex in the sequence to its successor in the sequence, with no repeated edges.

  • A directed path is simple if it has no repeated vertices.

  • A directed cycle is a directed path (with at least one edge) whose first and last vertices are the same.

  • A directed cycle is simple if it has no repeated vertices (other than the requisite repetition of the first and last vertices).

  • The length of a path or a cycle is its number of edges.

  • We say that a vertex w is reachable from a vertex v if there exists a directed path from v to w.

  • We say that two vertices v and w are strongly connected if they are mutually reachable: there is a directed path from v to w and a directed path from w to v.

  • A digraph is strongly connected if there is a directed path from every vertex to every other vertex.

  • A digraph that is not strongly connected consists of a set of strongly connected components, which are maximal strongly connected subgraphs.

  • A directed acyclic graph (or DAG) is a digraph with no directed cycles.

DiGraph: Set of vertices connect pairwise by directed edges

Adjacency List Representation

Java Implementation

public class diGraph {
  private final int V;
  private List<Integer>[] adj;

  public diGraph(int v) {
    this.V = v;
    adj = new List[v];
    for (int i = 0; i < V; i++) {
      adj[i] = new ArrayList<>();
    }
  }

  public void addEdge(int v, int w) {
    adj[v].add(w);
  }

  public Iterable<Integer> adj(int v) {
    return adj[v];
  }

  public static void main(String[] args) {}
}

  • Space Complexity : E+VE + V

  • Add Edge: 1

  • Edge between vv and ww is outdegree(v)outdegree(v)

  • Iterate over vertices adjacent to vv - outdegree(v)outdegree(v)

Depth First Search

Java implementation is same as undirected graph

Breadth First Search

Java implementation is same as undirected graph

Depth First Order

Depth-first search search visits each vertex exactly once. Three vertex orderings are of interest in typical applications:

  • Preorder: Put the vertex on a queue before the recursive calls.

  • Postorder: Put the vertex on a queue after the recursive calls.

  • Reverse postorder: Put the vertex on a stack after the recursive calls.

Java Implementation

/******************************************************************************
 *  Compilation:  javac DepthFirstOrder.java
 *  Execution:    java DepthFirstOrder digraph.txt
 *  Dependencies: Digraph.java Queue.java Stack.java StdOut.java
 *                EdgeWeightedDigraph.java DirectedEdge.java
 *  Data files:   https://algs4.cs.princeton.edu/42digraph/tinyDAG.txt
 *                https://algs4.cs.princeton.edu/42digraph/tinyDG.txt
 *
 *  Compute preorder and postorder for a digraph or edge-weighted digraph.
 *  Runs in O(E + V) time.
 *
 *  % java DepthFirstOrder tinyDAG.txt
 *     v  pre post
 *  --------------
 *     0    0    8
 *     1    3    2
 *     2    9   10
 *     3   10    9
 *     4    2    0
 *     5    1    1
 *     6    4    7
 *     7   11   11
 *     8   12   12
 *     9    5    6
 *    10    8    5
 *    11    6    4
 *    12    7    3
 *  Preorder:  0 5 4 1 6 9 11 12 10 2 3 7 8 
 *  Postorder: 4 5 1 12 11 10 9 6 0 3 2 7 8 
 *  Reverse postorder: 8 7 2 3 0 6 9 10 11 12 1 5 4 
 *
 ******************************************************************************/

/**
 *  The {@code DepthFirstOrder} class represents a data type for 
 *  determining depth-first search ordering of the vertices in a digraph
 *  or edge-weighted digraph, including preorder, postorder, and reverse postorder.
 *  <p>
 *  This implementation uses depth-first search.
 *  Each constructor takes &Theta;(<em>V</em> + <em>E</em>) time,
 *  where <em>V</em> is the number of vertices and <em>E</em> is the
 *  number of edges.
 *  Each instance method takes &Theta;(1) time.
 *  It uses &Theta;(<em>V</em>) extra space (not including the digraph).
 *  <p>
 *  For additional documentation,
 *  see <a href="https://algs4.cs.princeton.edu/42digraph">Section 4.2</a> of
 *  <i>Algorithms, 4th Edition</i> by Robert Sedgewick and Kevin Wayne.
 *
 *  @author Robert Sedgewick
 *  @author Kevin Wayne
 */
public class DepthFirstOrder {
    private boolean[] marked;          // marked[v] = has v been marked in dfs?
    private int[] pre;                 // pre[v]    = preorder  number of v
    private int[] post;                // post[v]   = postorder number of v
    private Queue<Integer> preorder;   // vertices in preorder
    private Queue<Integer> postorder;  // vertices in postorder
    private int preCounter;            // counter or preorder numbering
    private int postCounter;           // counter for postorder numbering

    /**
     * Determines a depth-first order for the digraph {@code G}.
     * @param G the digraph
     */
    public DepthFirstOrder(Digraph G) {
        pre    = new int[G.V()];
        post   = new int[G.V()];
        postorder = new Queue<Integer>();
        preorder  = new Queue<Integer>();
        marked    = new boolean[G.V()];
        for (int v = 0; v < G.V(); v++)
            if (!marked[v]) dfs(G, v);

        assert check();
    }

    /**
     * Determines a depth-first order for the edge-weighted digraph {@code G}.
     * @param G the edge-weighted digraph
     */
    public DepthFirstOrder(EdgeWeightedDigraph G) {
        pre    = new int[G.V()];
        post   = new int[G.V()];
        postorder = new Queue<Integer>();
        preorder  = new Queue<Integer>();
        marked    = new boolean[G.V()];
        for (int v = 0; v < G.V(); v++)
            if (!marked[v]) dfs(G, v);
    }

    // run DFS in digraph G from vertex v and compute preorder/postorder
    private void dfs(Digraph G, int v) {
        marked[v] = true;
        pre[v] = preCounter++;
        preorder.enqueue(v);
        for (int w : G.adj(v)) {
            if (!marked[w]) {
                dfs(G, w);
            }
        }
        postorder.enqueue(v);
        post[v] = postCounter++;
    }

    // run DFS in edge-weighted digraph G from vertex v and compute preorder/postorder
    private void dfs(EdgeWeightedDigraph G, int v) {
        marked[v] = true;
        pre[v] = preCounter++;
        preorder.enqueue(v);
        for (DirectedEdge e : G.adj(v)) {
            int w = e.to();
            if (!marked[w]) {
                dfs(G, w);
            }
        }
        postorder.enqueue(v);
        post[v] = postCounter++;
    }

    /**
     * Returns the preorder number of vertex {@code v}.
     * @param  v the vertex
     * @return the preorder number of vertex {@code v}
     * @throws IllegalArgumentException unless {@code 0 <= v < V}
     */
    public int pre(int v) {
        validateVertex(v);
        return pre[v];
    }

    /**
     * Returns the postorder number of vertex {@code v}.
     * @param  v the vertex
     * @return the postorder number of vertex {@code v}
     * @throws IllegalArgumentException unless {@code 0 <= v < V}
     */
    public int post(int v) {
        validateVertex(v);
        return post[v];
    }

    /**
     * Returns the vertices in postorder.
     * @return the vertices in postorder, as an iterable of vertices
     */
    public Iterable<Integer> post() {
        return postorder;
    }

    /**
     * Returns the vertices in preorder.
     * @return the vertices in preorder, as an iterable of vertices
     */
    public Iterable<Integer> pre() {
        return preorder;
    }

    /**
     * Returns the vertices in reverse postorder.
     * @return the vertices in reverse postorder, as an iterable of vertices
     */
    public Iterable<Integer> reversePost() {
        Stack<Integer> reverse = new Stack<Integer>();
        for (int v : postorder)
            reverse.push(v);
        return reverse;
    }


    // check that pre() and post() are consistent with pre(v) and post(v)
    private boolean check() {

        // check that post(v) is consistent with post()
        int r = 0;
        for (int v : post()) {
            if (post(v) != r) {
                StdOut.println("post(v) and post() inconsistent");
                return false;
            }
            r++;
        }

        // check that pre(v) is consistent with pre()
        r = 0;
        for (int v : pre()) {
            if (pre(v) != r) {
                StdOut.println("pre(v) and pre() inconsistent");
                return false;
            }
            r++;
        }

        return true;
    }

    // throw an IllegalArgumentException unless {@code 0 <= v < V}
    private void validateVertex(int v) {
        int V = marked.length;
        if (v < 0 || v >= V)
            throw new IllegalArgumentException("vertex " + v + " is not between 0 and " + (V-1));
    }

    /**
     * Unit tests the {@code DepthFirstOrder} data type.
     *
     * @param args the command-line arguments
     */
    public static void main(String[] args) {
        In in = new In(args[0]);
        Digraph G = new Digraph(in);

        DepthFirstOrder dfs = new DepthFirstOrder(G);
        StdOut.println("   v  pre post");
        StdOut.println("--------------");
        for (int v = 0; v < G.V(); v++) {
            StdOut.printf("%4d %4d %4d\n", v, dfs.pre(v), dfs.post(v));
        }

        StdOut.print("Preorder:  ");
        for (int v : dfs.pre()) {
            StdOut.print(v + " ");
        }
        StdOut.println();

        StdOut.print("Postorder: ");
        for (int v : dfs.post()) {
            StdOut.print(v + " ");
        }
        StdOut.println();

        StdOut.print("Reverse postorder: ");
        for (int v : dfs.reversePost()) {
            StdOut.print(v + " ");
        }
        StdOut.println();


    }

}

DI-Graph Applications

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