25期代码随想录算法训练营第三天 | 203.移除链表元素、707.设计链表、206.反转链表

203.移除链表元素

链接

使用虚拟头

# 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 = ListNode(next = head)
        cur = dummy

        while cur.next:
            if cur.next.val == val:
                cur.next = cur.next.next
            else:
                cur = cur.next
            
        return dummy.next

707.设计链表

链接

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

class MyLinkedList:

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

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

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

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


    def addAtTail(self, val: int) -> None:
        cur = self.dummy_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.dummy_head
        for i in range(index):
            cur = cur.next
        cur.next = ListNode(val = val, next = cur.next)
        self.size += 1

    def deleteAtIndex(self, index: int) -> None:
        if index < 0 or index >= self.size:
            return
        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

链接

# 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]:
        prev = None
        cur = head

        while cur:
            temp = cur.next
            cur.next = prev
            prev = cur
            cur = temp
        
        return prev