use node.val as the heap cmp key

```
# Definition for singly-linked list.
# class ListNode(object):
# def __init__(self, x):
# self.val = x
# self.next = None
import heapq
class Solution(object):
def mergeKLists(self, lists):
"""
:type lists: List[ListNode]
:rtype: ListNode
"""
root=ListNode(0)
rroot=root
heap=[]
for head in lists:
if not head:
continue
heapq.heappush(heap,(head.val,head))
while heap:
nsnode=heapq.heappop(heap)[1]
rroot.next=nsnode
rroot=rroot.next
nsnode=nsnode.next
if nsnode:
heapq.heappush(heap,(nsnode.val,nsnode))
rroot.next=None
return root.next
```