LeetCode 230. Kth Smallest Element in a BST

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?

思路：

二叉搜索树的中序遍历（LNR）就是按照从小到大顺序排的，所以遍历到N节点的时候就记个数，计数到k的时候就返回了。

代码：

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    int kthSmallest(TreeNode* root, int k) {
        if(root == NULL)
            return -1;
        int retVal = -1;
        int count = 0;
        kthSmallest1(root, k, count, retVal);
        return retVal;
    }
    void kthSmallest1(TreeNode* root, int k, int& count, int& retVal){
        if(count >= k)
            return;
        if(root->left != NULL)
            kthSmallest1(root->left, k, count, retVal);
        count ++;
        if(count == k){
            retVal = root->val;
            return;
        }
        if(root->right != NULL)
            kthSmallest1(root->right, k, count, retVal);
    }
};