105. Construct Binary Tree from Preorder and Inorder Traversal

我觉得我自己的递归写的还是非常不熟悉，这道题本应该是很简单的。。。

# 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
        """
        if len(preorder) == 0:
            return None
        if len(preorder) == 1:
            return TreeNode(preorder[0])
        root = TreeNode(preorder[0])
        root_index = inorder.index(root.val)
        root.left = self.buildTree(preorder[1:1+root_index], inorder[:root_index])
        root.right =  self.buildTree(preorder[1+root_index:], inorder[root_index+1:])
        return root