public void inorder(TreeNode root){
        if(root == null) return;
        inorder(root.left);
        if(pre == null){
            pre = root;
        }else{
            if(pre.val > root.val){
                if(first == null) first = pre;
                second = root;
            }
            pre = root;
        }
        inorder(root.right);
    }