//Recursive
public ArrayList<Integer> postorderTraversal(TreeNode root) {
    ArrayList<Integer> result = new ArrayList<Integer>();

    if (root == null) {
        return result;
    }

    result.addAll(postorderTraversal(root.left));
    result.addAll(postorderTraversal(root.right));
    result.add(root.val);

    return result;
}

//Iterative
public ArrayList<Integer> postorderTraversal(TreeNode root) {
    ArrayList<Integer> result = new ArrayList<Integer>();
    Stack<TreeNode> stack = new Stack<TreeNode>();
    TreeNode prev = null; // previously traversed node
    TreeNode curr = root;

    if (root == null) {
        return result;
    }

    stack.push(root);
    while (!stack.empty()) {
        curr = stack.peek();
        if (prev == null || prev.left == curr || prev.right == curr) { // traverse down the tree
            if (curr.left != null) {
                stack.push(curr.left);
            } else if (curr.right != null) {
                stack.push(curr.right);
            }
        } else if (curr.left == prev) { // traverse up the tree from the left
            if (curr.right != null) {
                stack.push(curr.right);
            }
        } else { // traverse up the tree from the right
            result.add(curr.val);
            stack.pop();
        }
        prev = curr;
    }

    return result;
}