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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?

题解：使用DFS，BTS的结构是root.left<root<root.right,采用中序遍历，可以得到k小的值。

code：

public class Solution {    private int value;	private int index;	public int kthSmallest(TreeNode root, int k) {		index = 0;		dfs(root, k);		return value;	}	public void dfs(TreeNode root, int k) {		if(root == null)			return;		dfs(root.left, k);		index++;		if (index == k) {			value = root.val;			return;		}		dfs(root.right, k);	}}

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