## Title description

Given a binary search tree, please find the k-th smallest node in it. For example, in (5,3,7,2,4,6,8), the value of the third small node is 4 in the order of node value.

## Idea analysis

The characteristics of the binary search tree are: the left node is larger than the root node, and the right node is smaller than the root node.

The result of the middle order traversal is: left-"middle-" right

The combination of the two is an ordered sequence.

## Code

```
import java.util.ArrayList;
/*
public class TreeNode {
int val = 0;
TreeNode left = null;
TreeNode right = null;
public TreeNode(int val) {
this.val = val;
}
}
*/
public class Solution {
ArrayList<TreeNode> list = new ArrayList<>();
TreeNode KthNode(TreeNode pRoot, int k)
{
KthHelper(pRoot);
if(k>=1&&k<=list.size()){
return list.get(k-1);
}
return null;
}
//使用中序遍历：左 中 右
void KthHelper(TreeNode root){
if(root!=null){
KthHelper(root.left);
list.add(root);
KthHelper(root.right);
}
}
}
```