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：
3
/
9 20
/
15 7

限制：
0 <= 节点个数 <= 5000

注意：本题与主站 105 题重复：https://leetcode-cn.com/problems/construct-binary-tree-from-preorder-and-inorder-traversal/

# 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:
        idx_map = {val: idx for idx, val in enumerate(inorder)}
        return self.build(0, len(inorder)-1, preorder, inorder, idx_map)
    def build(self, in_left, in_right, preorder, inorder, idx_map):
        if in_left > in_right:
            return None
        val = preorder.pop(0)
        root = TreeNode(val)
        index = idx_map[val]

        root.left = self.build(in_left, index-1, preorder, inorder, idx_map)
        root.right = self.build(index+1, in_right, preorder, inorder, idx_map)
        return root