我的方法没有达到预期的返回语句

Question

I have been staring at this for hours now and can't figure out what is going wrong,

I am trying to implement a method that determines whether or not a path exists between two vertices in a graph, using a depth-first search algorithm.

However, while it seems to succesfully find the vertex it needs to find, it doen't reach the return statement.

I've tried using different variables, printing stuff, changing orders, but can't get this to return true. Any help would be greatly appreciated!

public class Path<V,E> {

private HashMap<V,Boolean> visited = new HashMap<V,Boolean>();

public boolean pathExists(Graph<V,E> graph, V v1, V v2) {

    if (v1.equals(v2)) {
        System.out.println("v1 = v2");
        return true;
    }

    if (!visited.containsKey(v1)) {
        System.out.println("v1 discovered: "+v1);
        visited.put(v1, true);
    }

    for (V v : graph.getNeighbors(v1)) {

        if (v.equals(v2)) {
            visited.clear();
            System.out.println("v discovered: "+v);
            return true;
        }

        if (!visited.containsKey(v)) {
            visited.put(v,true);
            System.out.println("visited doesn't contain v: "+v);
            System.out.println("starting new search with "+v);
            pathExists(graph,v,v2);
        }

    }
    visited.clear();
    return false;
}



public static void main(String[] args) {

    MyGraph<String,String> mg = new MyGraph<String,String>();
    Path<String,String> path = new Path<String,String>();

    mg.insertVertex("utrecht");
    mg.insertVertex("amsterdam");
    mg.insertVertex("maastricht");

    mg.insertEdge("utrecht", "amsterdam", "route1", 2);
    mg.insertEdge("amsterdam", "maastricht", "route2", 1);

    boolean c = path.pathExists(mg, "utrecht", "maastricht");

    System.out.println(c);

}
}

This is the console output.

v1 discovered: utrecht<br>
visited doesn't contain v: amsterdam<br>
starting new search with amsterdam<br>
v discovered: maastricht<br>
false<br>

Since it is printing "v discovered: maastricht", I expect it to be returning true, but it returns false instead.

Answer 1

You have two main errors in your program.

1. You indeed return true when you have found your vertex, but you don't use the return value. Since your method is recursive, the return value is lost.

   Instead of

   pathExists(graph,v,v2);

   You should write

   if(pathExists(graph,v,v2)) return true;

2. You wrongly clear your visited map during the recursion.
   First of all, it is not needed at all except if you need to reuse your visited map after. But even in that case, it's better to clear it outside of your method.
   While the visited.clear() under if (v.equals(v2)) is not really an issue (apart from the previous remarks), the second one after your for loop will create bugs. For example, consider the following vertices with their adjacency lists:

    1 -> [2, 4] 2 -> [3] 4 -> [1, 5] 5 -> []

   Let's say that you look for vertex 5 and start from vertex 1 . You will visit vertex 2 and vertex 3 . Your visited map contains [1,2,3] . A the end of pathExists for vertex 3 , since you haven't found 5 and 3 as no neighbor, you clear your visited map. Then you visit vertex 4 , and since you have cleared your map, it only contains [4] . So you visit 1 again, and so on.

3. This is not really an error, but there is no need to create a Map<Integer, Boolean> . Just create a Set and check for the presence of the vertex.