230. kth smallest element in a bst

public int kthSmallest(TreeNode root, int k) {
	int cnt = vis(root.left);
	if(k == cnt+1){
		return root.val;
	}else if(k < cnt+1){
		从左子树找
		return kthSmallest(root.left, k);
	}else{
		//从右子树找
		return kthSmallest(root.right, k - cnt - 1);
	}
	
}
//计算节点个数
public int vis(TreeNode root) {
	if (root == null) {
		return 0;
	}
	return vis(root.left) + vis(root.right) + 1;
}