Binary Tree Postorder Traversal

Given a binary tree, return the postorder traversal of its nodes' values.

For example:

Given binary tree 

{1,#,2,3}

,

1
\
2
/
3

return 

[3,2,1]

.

Note: Recursive solution is trivial, could you do it iteratively?

public class BinaryTreePostorderTraversal {
public ArrayList<Integer> postorderTraversal(TreeNode<Integer> root) {
if(root == null){
return new ArrayList<Integer>();
}
ArrayList<Integer> left = postorderTraversal(root.left);
ArrayList<Integer> right = postorderTraversal(root.right);
left.addAll(right);
left.add(root.data);
return left;
}
}

或者：

public static ArrayList<Integer> postorderTraversal(TreeNode root){
ArrayList<Integer> list = new ArrayList<>();
if(root == null) return list;
postorderTraversalHelper(root,list);
return list;
}
private static void postorderTraversalHelper(TreeNode root, ArrayList<Integer> list){
if(root == null) return;
postorderTraversalHelper(root.left,list);
postorderTraversalHelper(root.right,list);
list.add(root.val);
}