比较基础的二叉树排序树插入，写了个递归。

# Definition for a binary tree node.# class TreeNode(object):#     def __init__(self, x):#         self.val = x#         self.left = None#         self.right = Noneclass Solution(object):    def insertIntoBST(self, root, val):        """        :type root: TreeNode        :type val: int        :rtype: TreeNode        """        if root is None:            return TreeNode(val)                if val < root.val:            root.left = self.insertIntoBST(root.left, val)        else:            root.right = self.insertIntoBST(root.right, val)                    return root