使用广度优先搜索：如何到达终点？

Question

I am not sure why my code is not returning the correct path vertices. It is returning [abc] instead of [acf],and I don't know why. Is there something I'm missing here or doing wrong in my algorithm? Note: getNeighbors(String vertex) returns the connecting edges for the vertex in its parameter.

This is the test: My code stops at "assertEquals("c", route.next())" because it is returning "b" instead of a "c". my code' current output is [abc] and the expected is [acf]

public class PathingTest {

    @Test
    public void testPathing(){
        Graph cycle = new Graph("graphs/cycle.json");
        Iterator<String> route = cycle.getRoute("d", "b").iterator();
        assertEquals("d",route.next());
        assertEquals("b",route.next());
        assertFalse(route.hasNext());

        Graph tree = new Graph("graphs/tree.json");
        route = tree.getRoute("a", "f").iterator();
        assertEquals("a",route.next());
        assertEquals("c", route.next());
        assertEquals("f", route.next());
        assertFalse(route.hasNext());

        Graph disconnected = new Graph("graphs/disconnected.json");
        assertEquals(null, disconnected.getRoute("a", "f"));
    }
}

Answer 1

The queue variable and the visited variable have different purposes, but in your case, they get updated in the same way, which is not correct.

Shortly, you add a node to the queue while processing its parent (which means that at some point in the future, that node is expecting to be processed too). Meanwhile, you only add a node to visited only after you have processed it (added its children to the queue).

Your while loop should look like this (please notice the position of the insertion into visited ).

while (!queue.isEmpty()) {
    String current = queue.remove();
    path.add(current);

    visited.add(current);

    if (current == end) {
        return path;
    }

    Iterator<String> neighbors = getNeighbors(start).iterator();
    while (neighbors.hasNext()) {
        String n = neighbors.next();

        if (!visited.contains(n)) {
            queue.add(n);
        }
    }
}