public class Solution {

    private TreeNode pre = null;
    /**
     * 给定一个二叉树根节点，请你判断这棵树是不是二叉搜索树
     *
     *
     * @param root TreeNode类
     * @return bool布尔型
     */
    public boolean isValidBST (TreeNode root) {
        if (null == root) {
            return false;
        }
        return inorder(root);
    }

    public boolean inorder(TreeNode curr) {
        if (null == curr) {
            return true;
        }
        boolean isLeftOK = inorder(curr.left);
        if (pre != null && pre.val >= curr.val) {
            return false;
        }
        pre = curr;
        boolean isRightOK = inorder(curr.right);
        return isLeftOK && isRightOK;
    }

}