由前序的特點，第一個必定是root結點

由中序的特點，根節點左側為左子樹，右側為右子樹。

遞迴流程為：

1）從preorder中找到根節點

2）從inorder中拆分出左右子樹

3）按長度在preorder中繼續找到左右子樹的preorder進行遞迴建樹

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
 

class Solution:
    def buildTree(self, preorder: List[int], inorder: List[int]) -> TreeNode:
        if not preorder:
            return None
        
        root = TreeNode(preorder[0])        
        left_inorder = inorder[: inorder.index(root.val)]
        right_inorder = inorder[inorder.index(root.val) + 1 :]
        
        l_left 
 
= len(left_inorder)
        left_preorder = preorder[1:l_left + 1]
        right_preorder = preorder[l_left + 1 :]
        
        root.left = self.buildTree(left_preorder, left_inorder)
        root.right = self.buildTree(right_preorder, right_inorder)
        
        return root