LeetCode--105--从前序与中序遍历序列构造二叉树（python）

 1 class Solution:
 2     def buildTree(self, preorder, inorder):
 3         """
 4         :type preorder: List[int]
 5         :type inorder: List[int]
 6         :rtype: TreeNode
 7         """
 8         def helper(in_left = 0, in_right = len(inorder)):
 9             nonlocal pre_idx
10             # if there is no elements to construct subtrees
11             if in_left == in_right:
12                 return None
13             
14             # pick up pre_idx element as a root
15             root_val = preorder[pre_idx]
16             root = TreeNode(root_val)
17 
18             # root splits inorder list
19             # into left and right subtrees
20             index = idx_map[root_val]
21 
22             # recursion 
23             pre_idx += 1
24             # build left subtree
25             root.left = helper(in_left, index)
26             # build right subtree
27             root.right = helper(index + 1, in_right)
28             return root
29         
30         # start from first preorder element
31         pre_idx = 0
32         # build a hashmap value -> its index
33         idx_map = {val:idx for idx, val in enumerate(inorder)} 
34         return helper()