技術標籤：leetcodeleetcodejava

Integer res = null;
    boolean flag =true;
    public boolean isValidBST(TreeNode root) {
        if (root == null) return true;
        isValidBST(root.left);
        if (res == null) res = root.val;
        else if (root.val <= res) flag = false;
        else {
            res =
 
 root.val;
        }
        isValidBST(root.right);
        return flag;
    }