class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> result = new ArrayList<>();
        if (root == null) {
            return result;
        }
        Deque<TreeNode> queue = new ArrayDeque<>();
        queue.add(root);
        while(!queue.isEmpty()) {
            TreeNode node = queue.pop();
            result.add(0,node.val);
            if (node.left != null) {
                queue.push(node.left);
            }
            if (node.right != null) {
                queue.push(node.right);
            }
        }
        return result;
    }
}