[Leetcode] Kth Smallest Element in a BST, Solution

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?

[Thoughts]
递归遍历二叉树，在遍历过程中算kth。


[Code]

1:    int kthSmallest(TreeNode* root, int k) {  
2:      int count = 0;  
3:        
4:      return kthSmallestImpl(root, count, k);  
5:        
6:    }  
7:      
8:    int kthSmallestImpl(TreeNode* root, int& count, int k) {  
9:      if(root == NULL) return -1;  
10:        
11:      int lr = kthSmallestImpl(root->left, count, k);  
12:      count++;  
13:      if(count == k) {  
14:        return root->val;  
15:      }  
16:        
17:      int rr = kthSmallestImpl(root->right, count, k);  
18:      return lr != -1 ? lr : rr;  
19:    }