private static <L> void traversalHelper(Tree<L> tree, List<Tree<L>> traversal, boolean preOrder) {
if (preOrder) traversal.add(tree);
for (Tree<L> child : tree.getChildren()) {
traversalHelper(child, traversal, preOrder);
}
if (!preOrder) traversal.add(tree);
}
public List<Tree<L>> getPostOrderTraversal() {
ArrayList<Tree<L>> traversal = new ArrayList<Tree<L>>();
traversalHelper(this, traversal, false);
return traversal;
}