已知列表nums，将其转化为二叉树。举例：

nums = [3,9,20,None,None,15,7]，转化为二叉树后，节点3的左子节点9，右子节点20，9的左右子节点都为None，20的左子节点15，右子节点7，参考附件图片1

二叉树定义：
class TreeNode:
def init(self, x):
self.val = x
self.left = None
self.right = None

请补全下面函数：
def list_to_binarytree(nums):
pass

class TreeNode:
def init(self, x):
self.val = x
self.left = None
self.right = None

def list_to_binarytree(nums):

# def level(index):
    # if index >= len(nums):# or nums[index] is None:
        # return None

    # print(nums[index])
    # root = TreeNode(nums[index])
    # root.left = level(index*2+1)
    # root.right = level(index*2+2)
    # return root
# return level(0)

def level(index):
if index >= len(nums):# or nums[index] is None:
return None

root = TreeNode(nums[index])
root.left = level(index*2+1)
root.right = level(index*2+2)
return root

def traverse_tree(root):
if root is not None:
print(root.val, end=' ')
if root:
if root.left is not None:
traverse_tree(root.left)
if root.right is not None:
traverse_tree(root.right)

if name == 'main':
nums = [3,9,20,None,None,15,7]
root = level(0)
traverse_tree(root)