代码随想录算法训练营day3|● 203.移除链表元素 ● 707.设计链表 ● 206.反转链表

学习资料：https://programmercarl.com/链表理论基础.html#链表的类型
可设置虚拟头结点 dummy_head
链表最后指向Null
一个节点包含值和索引

学习记录：
203.移除链表元素（基本ListNode(),cur.next, cur.next.val）

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# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:def removeElements(self, head: Optional[ListNode], val: int) -> Optional[ListNode]:dummy_head = ListNode(next=head)cur = dummy_headwhile cur.next:if cur.next.val == val:cur.next = cur.next.nextelse:cur = cur.nextreturn dummy_head.next

707.设计链表（找到索引对应值；在第一个元素前插入一个节点；在最后一个元素后面插入一个节点；在索引处插入一个节点；注意链表size变化）

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class ListNode:def __init__(self, val=0, next=None):self.val = valself.next = nextclass MyLinkedList:def __init__(self):self.dummpy_head = ListNode()self.size = 0def get(self, index: int) -> int:if index < 0 or index >= self.size:return -1cur = self.dummpy_head.nextfor i in range(index):cur = cur.nextreturn cur.valdef addAtHead(self, val: int) -> None:self.dummpy_head.next = ListNode(val, self.dummpy_head.next)self.size += 1def addAtTail(self, val: int) -> None:cur = self.dummpy_headwhile cur.next:cur = cur.nextcur.next = ListNode(val)self.size += 1def addAtIndex(self, index: int, val: int) -> None:if index < 0 or index > self.size:returncur = self.dummpy_headfor i in range(index):cur = cur.nextcur.next = ListNode(val, cur.next)self.size += 1def deleteAtIndex(self, index: int) -> None:if index<0 or index>=self.size:returncur = self.dummpy_headfor i in range(index):cur=cur.nextcur.next = cur.next.nextself.size -= 1# Your MyLinkedList object will be instantiated and called as such:
# obj = MyLinkedList()
# param_1 = obj.get(index)
# obj.addAtHead(val)
# obj.addAtTail(val)
# obj.addAtIndex(index,val)
# obj.deleteAtIndex(index)

206.翻转链表（双指针法）

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# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:def reverseList(self, head: Optional[ListNode]) -> Optional[ListNode]:cur = headpre = Nonewhile cur:temp = cur.nextcur.next = pre   # 翻转pre = curcur = tempreturn pre

PS:
第一次接触链表，很懵，感觉707题还要反复刷，另外可以用极端条件或画图来更直观判断操作或赋值怎么写
多云转晴，放假期间很难坚持学习啊，幸好有美食支撑（柴火鸡、煲仔饭、泡鸡爪）