输入：[8,5,1,7,10,12]输出：[8,5,10,1,7,null,12]

1 # Definition for a binary tree node. 2 # class TreeNode(object): 3 #     def __init__(self, x): 4 #         self.val = x 5 #         self.left = None 6 #         self.right = None 7  8 class Solution(object): 9     def bstFromPreorder(self, preorder):10         """11         :type preorder: List[int]12         :rtype: TreeNode13         """14         if len(preorder)==0: return None15         if len(preorder)==1: return TreeNode(preorder[0])16         17         root = TreeNode(preorder[0])18         idx=119         while idx