前言

• 一鼓作气把链表给刷完！！中等题困难题冲冲冲啊啊啊！

25. K 个一组翻转链表 - 力扣（LeetCode）

• 模拟

  • 

  • class Solution:def reverseKGroup(self, head: Optional[ListNode], k: int) -> Optional[ListNode]:# 翻转链表前k项def reverse(head, k):pre, cur = None, headwhile cur and k:temp = cur.nextcur.next = prepre = curcur = tempk -= 1return pre, head  # pre为头，head为尾dummy = ListNode(0, head)pre = cur = dummycount = k  # 用于重复计数while cur.next and count:count -= 1cur = cur.nextif count == 0:temp = cur.next  # 存一下段后节点pre.next, cur = reverse(pre.next, k)  # 连接段前+翻转cur.next = temp  # 连上段后节点pre = cur  # 更新pre指针count = k  # 恢复count继续遍历return dummy.next

 138. 随机链表的复制 - 力扣（LeetCode）

• 路飞的题解真是太强啦！！优雅清晰简洁

• 哈希表

  • """
    # Definition for a Node.
    class Node:def __init__(self, x: int, next: 'Node' = None, random: 'Node' = None):self.val = int(x)self.next = nextself.random = random
    """class Solution:def copyRandomList(self, head: 'Optional[Node]') -> 'Optional[Node]':if not head: return Nonedic = {}# 1. 复制各节点，并建立 “原节点 -> 新节点” 的 Map 映射cur = headwhile cur:dic[cur] = Node(cur.val)cur = cur.next# 2. 构建新节点的 next 和 random 指向cur = headwhile cur:dic[cur].next = dic.get(cur.next)dic[cur].random = dic.get(cur.random)cur = cur.next# 3. 返回新链表的头节点    return dic[head]

• 拼接 + 拆分

  • class Solution:def copyRandomList(self, head: 'Optional[Node]') -> 'Optional[Node]':if not head: return Nonedic = {}# 1. 复制各节点，并构建拼接链表cur = headwhile cur:temp = Node(cur.val)temp.next = cur.nextcur.next = tempcur = temp.next# 2. 构建各新节点的 random 指向cur = headwhile cur:if cur.random:cur.next.random = cur.random.nextcur = cur.next.next# 3. 拆分两链表cur = newhead = head.nextpre = headwhile cur.next:pre.next = pre.next.nextcur.next = cur.next.nextpre = pre.nextcur = cur.nextpre.next = None  # 单独处理原链表尾节点return newhead  # 返回新链表头节点

148. 排序链表 - 力扣（LeetCode）

• 归并排序（顶到底递归）

  • 参考路飞题解
  • 

  • class Solution:def sortList(self, head: Optional[ListNode]) -> Optional[ListNode]:if not head or not head.next: return head# 快慢指针分割链表slow, fast = head, head.nextwhile fast and fast.next:fast, slow = fast.next.next, slow.nextmid = slow.next  # 右半部分的头节点slow.next = None  # 断开两部分# 递归进行归并排序left = self.sortList(head)right = self.sortList(mid)# 合并左右两个链表dummy = cur = ListNode(0)while left and right:  # 根据大小依次插入新链表if left.val < right.val:cur.next = leftleft = left.nextelse:cur.next = rightright = right.nextcur = cur.nextcur.next = left if left else right  # 接上剩下的return dummy.next

• 归并排序（底到顶合并） 

  • 

  • class Solution:def sortList(self, head: ListNode) -> ListNode:# 合并两个有序链表def merge(head1, head2):dummy = cur = ListNode(0)while head1 and head2:if head1.val < head2.val:cur.next = head1head1 = head1.nextelse:cur.next = head2head2 = head2.nextcur = cur.nextcur.next = head1 if head1 else head2return dummy.next# 如果只有一个节点直接返回headif not head: return head# 统计链表长度lenth = 0cur = headwhile cur:cur = cur.nextlenth += 1# 开始循环合并dummy = ListNode(0, head)sublenth = 1while sublenth < lenth:pre, cur = dummy, dummy.nextwhile cur:head1 = curfor i in range(1, sublenth):if cur.next:cur = cur.nextelse:break  # 如果还没找到head2说明不用合并，下一轮head2 = cur.nextif not head2: break  # 空就不合并了cur.next = None  # 断开第一段后cur = head2for i in range(1, sublenth):if cur.next:cur = cur.nextelse:breaktemp = cur.next  cur.next = None  # 断开第二段后cur = tempmerged = merge(head1, head2)  # 合并pre.next = mergedwhile pre.next:pre = pre.next  # pre更新到合并后链表的最后pre.next = temp  # 重新连接第二段后# 下一轮合并sublenth *= 2return dummy.next

 23. 合并 K 个升序链表 - 力扣（LeetCode）

• 依次合并

  • class Solution:def mergeKLists(self, lists: List[Optional[ListNode]]) -> Optional[ListNode]:# 合并两个有序链表def merge(head1, head2):dummy = cur = ListNode(0)while head1 and head2:if head1.val < head2.val:cur.next = head1head1 = head1.nextelse:cur.next = head2head2 = head2.nextcur = cur.nextcur.next = head1 if head1 else head2return dummy.nextlenth = len(lists)if lenth == 0:return None# 每遍历一个链表就合并掉dummyhead = ListNode(0)for i in range(0, len(lists)):dummyhead.next = merge(dummyhead.next, lists[i])return dummyhead.next

•  分治合并

  • class Solution:def mergeKLists(self, lists: List[ListNode]) -> ListNode:# 如果输入为空，直接返回空if not lists:return # 获取链表列表的长度n = len(lists)# 调用递归函数进行合并return self.merge(lists, 0, n-1)def merge(self, lists, left, right):# 当左右指针相等时，表示只有一个链表，直接返回该链表if left == right:return lists[left]# 计算中间位置mid = left + (right - left) // 2# 递归地合并左半部分和右半部分的链表l1 = self.merge(lists, left, mid)l2 = self.merge(lists, mid+1, right)# 调用合并两个有序链表的函数return self.mergeTwoLists(l1, l2)def mergeTwoLists(self, l1, l2):# 若其中一个链表为空，则直接返回另一个链表if not l1:return l2if not l2:return l1# 比较两个链表头结点的大小，选择较小的作为新链表的头结点if l1.val < l2.val:l1.next = self.mergeTwoLists(l1.next, l2)return l1else:l2.next = self.mergeTwoLists(l1, l2.next)return l2

•  最小堆

  • """
    假设有3个有序链表分别是：1->4->5, 1->3->4, 2->6。
    初始时，最小堆为空。我们依次将(1,0),(1,1),(2,2)加入最小堆。
    然后不断弹出最小值(1,0),(1,1),(2,2)，加入到结果链表中，
    并将对应链表的下一个节点值和索引加入最小堆，直到最小堆为空。
    最终得到的合并后的链表为1->1->2->3->4->4->5->6
    """
    class Solution:def mergeKLists(self, lists: List[ListNode]) -> ListNode:import heapq# 创建虚拟节点dummy = ListNode(0)p = dummyhead = []# 遍历链表数组for i in range(len(lists)):if lists[i] :# 将每个链表的头结点值和索引加入到最小堆中heapq.heappush(head, (lists[i].val, i))lists[i] = lists[i].nextwhile head:# 弹出最小堆中的值和对应的索引val, idx = heapq.heappop(head)# 创建新节点并连接到结果链表上p.next = ListNode(val)p = p.next# 如果该链表还有剩余节点，则将下一个节点的值和索引加入到最小堆中if lists[idx]:heapq.heappush(head, (lists[idx].val, idx))lists[idx] = lists[idx].next# 返回合并后的链表return dummy.next

146. LRU 缓存 - 力扣（LeetCode）

• 哈希 + 双向链表

  • 借用灵神题解的图，really good
  • 

  • class Node:# 提高访问属性的速度，并节省内存__slots__ = 'prev', 'next', 'key', 'value'def __init__(self, key=0, value=0):# self.prev = None# self.next = Noneself.key = keyself.value = valueclass LRUCache:def __init__(self, capacity: int):self.capacity = capacityself.dummy = Node()  # 哨兵节点self.dummy.prev = self.dummyself.dummy.next = self.dummyself.key_to_node = dict()def get_node(self, key: int) -> Optional[Node]:if key not in self.key_to_node:  # 没有这本书return Nonenode = self.key_to_node[key]  # 有这本书self.remove(node)  # 把这本书抽出来self.push_front(node)  # 放在最上面return nodedef get(self, key: int) -> int:node = self.get_node(key)return node.value if node else -1def put(self, key: int, value: int) -> None:node = self.get_node(key)if node:  # 有这本书node.value = value  # 更新 valuereturnself.key_to_node[key] = node = Node(key, value)  # 新书self.push_front(node)  # 放在最上面if len(self.key_to_node) > self.capacity:  # 书太多了back_node = self.dummy.prevdel self.key_to_node[back_node.key]self.remove(back_node)  # 去掉最后一本书# 删除一个节点（抽出一本书）def remove(self, x: Node) -> None:x.prev.next = x.nextx.next.prev = x.prev# 在链表头添加一个节点（把一本书放在最上面）def push_front(self, x: Node) -> None:x.prev = self.dummyx.next = self.dummy.nextx.prev.next = xx.next.prev = x# Your LRUCache object will be instantiated and called as such:
    # obj = LRUCache(capacity)
    # param_1 = obj.get(key)
    # obj.put(key,value)

后言

• 链表终于结束了！后面这几道真是难到我了，基本都是CV写出来的，还是得多沉淀啊 ！休息一下，晚上干点活了，不然新学期要被老板骂骂