[LeetCode]Kth Smallest Element in a BST

public class Solution {
TreeNode result = new TreeNode(0);
public int kthSmallest(TreeNode root, int k) {
helper(root, k);
return result.val;
}
public int helper(TreeNode root, int k) {
if (root == null) {
return 0;
}
int left = helper(root.left, k);
if (left == k - 1) {
result = root;
}
int right = helper(root.right, k - left - 1);
return left + right + 1;
}
}