思路：遞迴，易知前序的第0號元素為樹的根，在中序裡找這個根，然後以該根為界將中序陣列劃分成左右兩半，依次遞迴進行。難點在於每次還要根據中序的劃分來將前序陣列也劃分。

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    def buildTree(self, preorder, inorder)
 
:
        """
        :type preorder: List[int]
        :type inorder: List[int]
        :rtype: TreeNode
        """


        if len(preorder) == 0:
            return None

        root = TreeNode(preorder[0])
        mid = inorder.index(preorder[0])
        #以mid為界劃分中序
        in_left = inorder[:mid]
        in_right = inorder[mid+1
 
:]
        #根據中序劃分前序
        pre_left = preorder[1:len(in_left)+1]
        pre_right = preorder[len(in_left)+1:]
        #遞迴左右子樹
        root.left = self.buildTree(pre_left, in_left)
        root.right = self.buildTree(pre_right, in_right)

        return root