后序遍历：左右根，从叶子结点开始

//递归
public static void postOrder(TreeNode root){
    if(root != null){
    	postOrder(root.lchild);
		postOrder(root.rchild);
    	System.out.println(root.data + " ");
    }
}

//后序遍历---非递归
public List<TreeNode> postOrder(TreeNode root){
	while(root!=null || !stack.isEmpty()){
		while(root!=null){
			list.add(root);
			static.push(root);
			root = root.right;
		}
		TreeNode node = stack.pop();
		root = node.left;
	}
	Conlections.reverse(list);
	return list;
}