这题跟144和94一样了，但这题是hard。递归写法都是一致的，这里贴一下迭代写法，摘自https://discuss.leetcode.com/topic/30632/preorder-inorder-and-postorder-iteratively-summarization/2
很多人说他在cheating，我模拟了一下，好像没什么问题，不深究了。

//Post Order Traverse
public List<Integer> postorderTraversal(TreeNode root) {
    LinkedList<Integer> result = new LinkedList<>();
    Deque<TreeNode> stack = new ArrayDeque<>();
    TreeNode p = root;
    while(!stack.isEmpty() || p != null) {
        if(p != null) {
            stack.push(p);
            result.addFirst(p.val);  // Reverse the process of preorder
            p = p.right;             // Reverse the process of preorder
        } else {
            TreeNode node = stack.pop();
            p = node.left;           // Reverse the process of preorder
        }
    }
    return result;
}

跟preorder对比：

    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> result = new ArrayList<>();
        Deque<TreeNode> stack = new ArrayDeque<>();
        TreeNode p = root;
        while(!stack.isEmpty() || p != null) {
            if(p != null) {
                stack.push(p);
                result.add(p.val);  // Add before going to children
                p = p.left;
            } else {
                TreeNode node = stack.pop();
                p = node.right;
            }
        }
        return result;
    }