Given a binary search tree, write a function ​​kthSmallest​​ to find the kth smallest element in it.

Note: 
You may assume k is always valid, 1 ≤ k ≤ BST's total elements.

Example 1:

Input: root = [3,1,4,null,2], k = 1
   3
  / \
 1   4
  \
   2
Output: 1

Example 2:

Input: root = [5,3,6,2,4,null,null,1], k = 3
       5
      / \
     3   6
    / \
   2   4
  /
 1
Output: 3

Follow up:
What if the BST is modified (insert/delete operations) often and you need to find the kth smallest frequently? How would you optimize the kthSmallest routine?

题解：

注意返回时改变n的值，不然永远返回根节点值。

class Solution {
public:
  int n = 1;
  void visitTree(TreeNode *t, int k, int &ans) {
    if (t->left != NULL) {
      visitTree(t->left, k, ans);
    }
    if (n == k) {
      ans = t->val;
      n++;
      return;
    }
    n++;
    if (t->right != NULL) {
      visitTree(t->right, k, ans);
    }
  }
  int kthSmallest(TreeNode* root, int k) {
    int ans = 0;
    visitTree(root, k, ans); 
    return ans;
  }
};