java depth first search

Question

This is my implementation for the reachable() of a graph using depth first search. It looks for a possible path from vertex 1 (v1) to another vertex (v2) I got some results correctly and some are wrong. I have tried as many ways as I can to debug, but I couldn'd figure out where is wrong. Any help is appreciated. Thank you

public boolean isReachable(V v1, V v2) {
            Stack<V> path = new Stack<V>();
            return isReachableHelper(v1, v2, path);
        }

}

public boolean isReachableHelper(V v1, V v2, Stack<V> path){
        path.push(v1);
        v1.setVisited(true); //set the vertex as being visited

        if (v1.vertex().equals(v2.vertex())){ //check whether vertex' values are equal
            return true;
        }

            //loop through all vertex that are neighbors of v1
        for (V neighbor : super.neighbors(v1)){ 
            if (!neighbor.visited ){ //if the neighbor is not visited before
                return isReachableHelper(neighbor, v2, path); //recursive call
            }
        }

        path.pop(); //no path was found
        return false;
    }

tobias_k · Accepted Answer · 2013-03-14 21:38:28Z

2

The Problem: In your for loop, you only ever expand the first non-visited neighbor node and then immediately return the value of that recursive call. I.e., if no path can be found via the first neighbor of v1, you just 'give up' without looking at the other neighbors.

Instead, you have to try all neighbor nodes until you find one for which the recursive call returns true. In this case, you have found a path and you can return true; otherwise, your method correctly pops the last node from the path and returns false (backtracking).

for (V neighbor : super.neighbors(v1)){ 
    if (!neighbor.visited ){ //if the neighbor is not visited before
        if (isReachableHelper(neighbor, v2, path)) { //recursive call
            return true; // we found a path!
        }
    }
}

edited Mar 14, 2013 at 21:38

answered Mar 14, 2013 at 21:32

tobias_k

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4 Comments

Mikeb Over a year ago

i was just writing up the same thing.

tobias_k Over a year ago

@Mikeb Sorry about that! ^^; There really should be an indicator for answers-in-progress...

Mikeb Over a year ago

nah, I like your explanation better than where I was going anyway.

kaboom Over a year ago

perfect explanation. Thank you

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