class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        LinkedList<TreeNode> stk = new LinkedList<>();
        TreeNode p = root;
        TreeNode r = null;
        List<Integer> res = new ArrayList<>();
        while (p != null || !stk.isEmpty()) {
            while (p != null) {
                stk.push(p);
                p = p.left;
            }
            p = stk.peek();
            if (p.right == null || p.right == r) {
                res.add(p.val);
                r = p;
                stk.pop();
                p = null;
            } else {
                p = p.right;
            }
        }
        return res;
    }
}