21/23. Merge 2/k Sorted Lists

合并k个链表，使用堆结构。

    def mergeKLists(self, lists):
        """
        :type lists: List[ListNode]
        :rtype: ListNode
        """
        import heapq
        pq = []
        for i in range(len(lists)):
            if lists[i]:
                # 此处调用.val,注意需要先判定node不为空
                heapq.heappush(pq, (lists[i].val, lists[i]))

        dummy = ListNode(-1)
        cur = dummy
        while pq:
            pair = heapq.heappop(pq)
            _, root = pair
            cur.next = root
            cur = cur.next
            if root.next:
                heapq.heappush(pq, (root.next.val, root.next))
        return dummy.next