【剑指offer】二叉查找树的第K个结点（python）

1. 二叉搜索树的中序(左中右)遍历结果就是升序序列，因为其左节点都小于根节点，右结点都大于根节点。
2. 注意边界值，0<k<=length

   # -*- coding:utf-8 -*-
   # class TreeNode:
   #     def __init__(self, x):
   #         self.val = x
   #         self.left = None
   #         self.right = None
   class Solution:
    # 返回对应节点TreeNode
    arr = []
    def KthNode(self, pRoot, k):
        # write code here
        if not pRoot: 
            return None
        self.arr = []
        self.midTraversal(pRoot)
        length = len(self.arr)
        if k <= length and k > 0:
            return self.arr[k-1]
        else:
            return None
    def midTraversal(self,root):
        if not root: 
            return None
        self.midTraversal(root.left)
        self.arr.append(root)
        self.midTraversal(root.right)