[LeetCode] 333. Largest BST Subtree

Given a binary tree, find the largest subtree which is a Binary Search Tree (BST), where largest means subtree with largest number of nodes in it.

Note:
A subtree must include all of its descendants.
Here's an example:

    10
    / \
   5  15
  / \   \ 
 1   8   7

The Largest BST Subtree in this case is the highlighted one. 
The return value is the subtree's size, which is 3.

Code:

   public int largestBSTSubtree(TreeNode root) {

        if (isValid(root,Integer.MAX_VALUE,Integer.MIN_VALUE)){

            return count(root);

        }

        return Math.max(largestBSTSubtree(root.left),largestBSTSubtree(root.right));

    }

    private boolean isValid(TreeNode root,int max,int min){

        if (root == null) return true;

        if (root.val > min && root.val < max){

            return isValid(root.left,root.val,min) && isValid(root.right,max,root.val);

        }

        return false;

    }

    private int count(TreeNode root){

        if (root == null) return 0;

        return 1 + count(root.left) + count(root.right);

    }