leetcode 230. kth smallest element in a bst

class  {
    
    public int count = 0; 
    public int res = 0;
    
    public int kthSmallest(TreeNode root, int k) {
        if (root == null) {
            return 0;
        }
        kthSmallest(root.left, k);    // 递归访问左子树
        count++;                      // 访问根
        if (count == k) {
            res = root.val;
            return res;
        }
        kthSmallest(root.right, k);   // 访问右子树
        return res;
    }   
}