Problem: Find all the vertices connected to a given vertex.

Class:

public class DepthFirstSearch
{
  private boolean[] marked;
  private int count;
  public DepthFirstSearch(Graph G, int s)
  {
    marked = new boolean[G.V()];
    dfs(G, s);
  }
  private void dfs(Graph G, int v)
  {
    marked[v] = true;
    count++;
    for (int w : G.adj(v))
      if (!marked[w]) dfs(G, w);
  }
  public boolean marked(int w)
  { return marked[w]; }
  public int count()
  { return count; }
}

Is vertex v is connected to s in graph G?

      DepthFirstSearch g = new DepthFirstSearch(G, s);
      boolean answer = g.marked(w);