【LEETCODE】105-Construct Binary Tree from Preorder and Inorder Traversal

Given preorder and inorder traversal of a tree, construct the binary tree.
Note:
You may assume that duplicates do not exist in the tree.

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

class Solution(object):
    def buildTree(self, preorder, inorder):
        """
        :type preorder: List[int]
        :type inorder: List[int]
        :rtype: TreeNode
        """
        
        location={}
        for i,num in enumerate(inorder):
            location[num]=i             
        
        return self.buildtreehelp(location,preorder,inorder,0,0,len(inorder))
        
    def buildtreehelp(self,location,preorder,inorder,pre_start,in_start,in_end):
        
        if in_start==in_end:
            return None
        
        root=TreeNode(preorder[pre_start])      #preorder     root
        
        i=location[preorder[pre_start]]         #  root inorder   
        
        root.left = self.buildtreehelp(location, preorder, inorder, pre_start + 1, in_start, i)       
            #root.left preorder inorder      ，   memory，        
        root.right = self.buildtreehelp(location, preorder, inorder, pre_start + 1 + i - in_start, i + 1, in_end)
        
        return root