剑指 Offer 07. 重建二叉树

输入某二叉树的前序遍历和中序遍历的结果，请重建该二叉树。假设输入的前序遍历和中序遍历的结果中都不含重复的数字。

例如，给出

前序遍历 preorder = [3,9,20,15,7]
中序遍历 inorder = [9,3,15,20,7]
返回如下的二叉树：

限制：

• 0 <= 节点个数 <= 5000

# 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: List[int], inorder: List[int]) -> TreeNode:
        if len(preorder) == 0:
            return None
        else:
            root = TreeNode(preorder[0])
            index = inorder.index(preorder[0])
            leftin = inorder[:index]
            rightin = inorder[index+1:]
            if index != -1:
                root.left = self.buildTree(preorder[1:1+len(leftin)], leftin)
                root.right = self.buildTree(preorder[1+len(leftin):], rightin)
            return root