1.合併 K 個升序連結串列
題目連結：https://leetcode.cn/problems/merge-k-sorted-lists/
給定一個連結串列陣列，每個連結串列都已經按升序排列。請將所有連結串列合併到一個升序連結串列中，返回合併後的連結串列。
class Solution:
def mergeKLists(self, lists: List[Optional[ListNode]]) -> Optional[ListNode]:
import heapq
dummy = ListNode()
curr = dummy
heap = []
for i, lst in enumerate(lists):
if lst:
heapq.heappush(heap, (lst.val, i))
while heap:
val, index = heapq.heappop(heap)
curr.next = ListNode(val)
curr = curr.next
if lists[index].next:
lists[index] = lists[index].next
heapq.heappush(heap, (lists[index].val, index))
return dummy.next
2. 兩兩交換連結串列中的節點
題目連結：https://leetcode.cn/problems/swap-nodes-in-pairs/
給定一個連結串列，兩兩交換其中相鄰的節點，並返回交換後連結串列的頭節點。
class Solution:
def swapPairs(self, head: Optional[ListNode]) -> Optional[ListNode]:
dummy = ListNode(0)
dummy.next = head
prev = dummy
while head and head.next:
first = head
second = head.next
prev.next = second
first.next = second.next
second.next = first
prev = first
head = first.next
return dummy.next