判断是否平衡二叉树

public boolean isValidBST(TreeNode root) {
        Stack<TreeNode> stack = new Stack<>();
        long tmp = Long.MIN_VALUE;
        while(!stack.isEmpty()||root!=null){
            while(root!=null){
                stack.push(root);
                root = root.left;
            }
            if(!stack.isEmpty()){
                root = stack.pop();
                //判断当前结点的值是否大于它的上一个入栈节点
                if(root.val<=tmp){
                    return false;
                }
                tmp = root.val * 1L;

                root = root.right;
            }
        }
        return true;
    }