Question

In the code below, the nodes of a postorder tree traversal is always printed
I m wondering if there is a way to store these nodes in the postorder sequence in an array
Do i have to do the traversal in a iterative way?

public static void postOrder(TreeNode root) {
  if (root != null) {
    postOrder(root.left);
    postOrder(root.right);
    System.out.print(root.item + "  ");
  } else {
    return;
  }
}

Answer 1

Just pass a List along with that method. Untested pseudo-ish code:

class Tree {

    private TreeNode root;

    // ...

    public List<TreeNode> postOrder() {
        List<TreeNode> nodes = new ArrayList<TreeNode>();  
        fillList(root, nodes);
        return nodes;
    }

    private void fillList(TreeNode node, List<TreeNode> nodeList) { // private!
        if(node != null) {
            fillList(node.left, nodeList);
            fillList(node.right, nodeList);
            nodeList.add(node);
        }
    }
}