链表203|707|206

203移除链表元素

# 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_node = ListNode()
        dummy_node.next = head
        current = dummy_node
        while current.next:
            if current.next.val == val:
                current.next = current.next.next
            else:
                current = current.next
        return dummy_node.next

707设计链表

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

    def __init__(self):
        self.dummy_head = ListNode()
        self.size = 0

    def get(self, index: int) -> int:
        if index < 0 or index >= self.size:
            return -1
        cur = self.dummy_head
        for i in range(index + 1):
            cur = cur.next
        return cur.val

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

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

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

    def deleteAtIndex(self, index: int) -> None:
        if 0 <= index and index < self.size:
            cur = self.dummy_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反转链表

# 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
            pre = cur
            cur = temp
        return pre