Given a binary search tree, find a small node of the k them. For example, (5,3,7,2,4,6,8), sorted by the size of the small value of the third node is node 4

```
#-*- coding:utf-8 -*-
class TreeNode:
def __init__(self,x):
self.val = x
self.left=None
self.right =None
class Solution:
def KthNode(self,pRoot,k):
retList=[]
def preOrder(pRoot):
if pRoot==None:
return None
preOrder(pRoot.left)
retList.append(pRoot)
preOrder(pRoot.right)
preOrder(pRoot)
if len(retList)<k or k <1:
return None
return retList[k-1]
```