算法:二叉搜索树的第k个结点

public class Solution {
    int index = 0;

    TreeNode KthNode(TreeNode root, int k) {
        if (root != null) {
            TreeNode node = KthNode(root.left, k);
            if (node != null)
                return node;
            index++;
            if (index == k)
                return root;
            node = KthNode(root.right, k);
            if (node != null)
                return node;
        }
        return null;
    }
}