just a simple solution, took 105msec = { merge - sort - convert }

I think this approach takes O(nlogn) average time if a system has enough memory.

```
class Solution(object):
def mergeKLists(self, lists):
merged_list = []
# merge into one list: O(n)
for list_k in lists:
node = list_k
while node != None:
merged_list.append(node.val)
node = node.next
if len(merged_list) < 1:
return None
# sort in a list: O(nlogn)
sorted_list = sorted(merged_list)
# convert python list to ListNode: O(n)
head = prev = ListNode(sorted_list[0])
for i in range(1, len(sorted_list)):
newnode = ListNode(sorted_list[i])
prev.next = newnode
prev = newnode
return head
```