Description:
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.
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?
Link:
https://leetcode.com/problems/kth-smallest-element-in-a-bst/description/
解题方法：
用中序遍历整个树，因为会优先遍历到最小的节点，然后再--k，看在哪个节点上k == 0，则用全局变量记录这个节点。
Time Complexity：
O(N)
完整代码：
int kthSmallest(TreeNode* root, int k)
{
int result = 0;
bool find = false;
helper(root, k, result, find);
return result;
}
void helper(TreeNode* root, int &k, int &result, bool &find)
{
if(!root || find)
return;
helper(root->left, k, result, find);
if(!--k)
{
find = true;
result = root->val;
}
helper(root->right, k, result, find);
}
