construct binary tree from preorder and inorder traversal

# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None
class (object):
    def buildTree(self, preorder, inorder):
        """
        :type preorder: List[int]
        :type inorder: List[int]
        :rtype: TreeNode
        """
        if not inorder:
            return None
        root = TreeNode(preorder.pop(0))
        index = inorder.index(root.val)
        root.left = self.buildTree(preorder, inorder[:index])
        root.right = self.buildTree(preorder, inorder[index+1:])
        return root

# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None
class (object):
    def buildTree(self, preorder, inorder):
        """
        :type preorder: List[int]
        :type inorder: List[int]
        :rtype: TreeNode
        """
        return self.helper(preorder, inorder, 0, len(inorder) - 1, 0, len(inorder) - 1)
    
    def helper(self, preorder, inorder, k, l, i, j):
        if k > len(preorder) - 1 or i > j:
            return None
        
        rootValue = preorder[k]
        root = TreeNode(rootValue)
        inorderIndex = inorder.index(rootValue)
        root.left = self.helper(preorder, inorder, k+1, k+inorderIndex-i-1, i, inorderIndex-1)
        root.right = self.helper(preorder, inorder, k+inorderIndex+1-i, l, inorderIndex+1, j)
        
        return root