Recursive Tree Traversal Method With Return Type Array

问题

Is there a way to recursively traverse a tree and return an array that is scoped to that recursive method?

So I recently answered someone else's question on this topic. That question can be found here: SO Question. My solution uses an array outside of the scope of the recursion, and therefore the method cannot (or at least probably should not) return the array. However, is there a way to write a recursive method for traversing trees such that it returns an array? Even writing an initial method that calls the recursive one would be fine, but I can't think of a good way to do this.

Here's the code that I suggested before:

private List nodeValues = new ArrayList();

public void traversePreRecursive(BinarySearchTreeNode node) 
{
    if (node != null)
    {
        nodeValues.add(node.getValue());
        traversePreRecursive(node.getLeft());
        traversePreRecursive(node.getRight());
    }
}

As you can see the ArrayList is outside of the scope of the recursion - And therefore returning it doesn't make a lot of sense. Is there a better way to do this?

回答1:

public static List traversePreRecursive(Node node) {
    if (node == null) return new ArrayList();

    List nodeValues = new ArrayList();
    nodeValues.add(node.getValue());
    nodeValues.addAll(traversePreRecursive(node.getLeft()));
    nodeValues.addAll(traversePreRecursive(node.getRight()));

    return nodeValues;
}

回答2:

There is an alternative, but it involves two passes over the tree. You would only employ this alternative if the array operations in my first answer were giving you grief. This approach starts by providing an index for each of the nodes (the index() method) -- basically working out which element of the array a node should occupy before we actually create the array. This also gives me a count of nodes (size). I then allocate an array (list) big enough to hold all the nodes and pass it into a method (addToList) that copies the node-references into the previously identified element in the array.

public static List<Node> getNodes(Node a) {
    int size = index(a, 0);
    List<Node> list = new ArrayList<Node>(size);
    addToList(a, list);
    return list;
}

private static int index(Node node, int index) {
    if (node == null) return index;

    node.setIndex(index);
    int iLeft = index(node.getLeft(), index++);
    int iRight = index(node.getRight(), iLeft++);

    return iRight + 1;
}

private static void addToList(Node node, List<Node> list) {
    if(node == null) return;
    list.add(node.getIndex(), node);
    addToList(node.getLeft(), list);
    addToList(node.getRight(), list);
}

回答3:

In c you can have static function variables,(Ie, adding a value to a list in one iteration of a function means that that value will be in the list in the next iteration--if the list is static) but using them isn't the best (most optimal) solution for the problem you are suggesting. So, I think you are searching for static variables, but this isn't an appropriate case to use them.

来源：https://stackoverflow.com/questions/16996314/recursive-tree-traversal-method-with-return-type-array

标签

java

arrays

recursion

binary-tree

traversal