leetcode 333. Largest BST Subtree

leetcode 333. Largest BST Subtree

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public int largestBSTSubtree(TreeNode root) {
        if(root==null) return 0;
        if(root.left==null&&root.right==null) return 1;
        if(isValid(root, null, null)) return countNode(root);
        return Math.max(largestBSTSubtree(root.left),largestBSTSubtree(root.right));
    }
    
    private boolean isValid(TreeNode root, Integer min, Integer max){
        if(root==null) return true;
        if(min!=null&&min>=root.val) return false;
        if(max!=null&&max<=root.val) return false;
        return isValid(root.left, min, root.val)&&isValid(root.right,root.val,max);
    }
    
    private int countNode(TreeNode root){
        if(root==null) return 0;
        if(root.left==null&&root.right==null) return 1;
        return 1 + countNode(root.left) + countNode(root.right);
    }
}