24. 两两交换链表中的节点

代码

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


class Solution:
def swapPairs(self, head: Optional[ListNode]) -> Optional[ListNode]:

    dummy_head= ListNode(next=head)

    cur = dummy_head

    while cur.next != None and cur.next.next!= None:

        temp = cur.next

        temp1 = cur.next.next.next

        cur.next = cur.next.next

        cur.next.next = temp

        temp.next = temp1

        cur = cur.next.next

    return dummy_head.next

19.删除链表的倒数第N个节点

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 removeNthFromEnd(self, head: Optional[ListNode], n: int) -> Optional[ListNode]:
        
        dummpy_head = ListNode(next=head)
        fast = dummpy_head
        slow = dummpy_head
        n += 1

        for i in range(n):
            if fast.next:
               fast = fast.next
            else:
                return dummpy_head.next.next #
        
        while fast.next:
            slow = slow.next
            fast = fast.next
        
        slow.next.next = slow.next.next.next

        return dummpy_head.next