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）

点击查看代码

# 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_head
        while cur.next:
            if cur.next.val == val:
                cur.next = cur.next.next
            else:
                cur = cur.next
        return 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）

点击查看代码

class ListNode:
    def __init__(self, val=0, next=None):
        self.val = val
        self.next = next

class MyLinkedList:
    def __init__(self):
        self.dummpy_head = ListNode()
        self.size = 0

    def get(self, index: int) -> int:
        if index < 0 or index >= self.size:
            return -1

        cur = self.dummpy_head.next
        for i in range(index):
            cur = cur.next
        return cur.val


    def addAtHead(self, val: int) -> None:
        self.dummpy_head.next = ListNode(val, self.dummpy_head.next)
        self.size += 1


    def addAtTail(self, val: int) -> None:
        cur = self.dummpy_head
        while cur.next:
            cur = cur.next
        cur.next = ListNode(val)
        self.size += 1


    def addAtIndex(self, index: int, val: int) -> None:
        if index < 0 or index > self.size:
            return
        cur = self.dummpy_head
        for i in range(index):
            cur = cur.next
        cur.next = ListNode(val, cur.next)
        self.size += 1

    def deleteAtIndex(self, index: int) -> None:
        if index<0 or index>=self.size:
            return
        cur = self.dummpy_head
        for i in range(index):
            cur=cur.next
        cur.next = cur.next.next
        self.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）

点击查看代码

# 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 = head
        pre = None
        while cur:
            temp = cur.next
            cur.next = pre   # 翻转
            pre = cur
            cur = temp
        return pre