LC 23合并 K 个升序链表Merge k Sorted Lists　困难

将 k 条升序链表合并为一条升序链表并返回头节点。

思路 小根堆存 k 个当前节点，弹最小接结果、推其后继入堆。时间 O(N log k)。

class Solution:
    def mergeKLists(self, lists):
        dummy = cur = ListNode()
        heap = [(n.val, i, n)
                for i, n in enumerate(lists) if n]
        heapq.heapify(heap)
        while heap:
            val, i, node = heapq.heappop(heap)
            cur.next = node
            cur = cur.next
            nxt = node.next
            if nxt:
                heapq.heappush(heap, (nxt.val, i, nxt))
        return dummy.next