25期代码随想录算法训练营第三天 | 链表 part 2

24. 两两交换链表中的节点

链接

双指针

# Definition for singly-linked list.
# 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 = ListNode(next = head)

        cur = dummy

        while cur.next and cur.next.next:
            temp1 = cur.next
            temp3 = cur.next.next.next

            cur.next = temp1.next 
            cur.next.next = temp1
            temp1.next = temp3
            cur = temp1
        
        return dummy.next

19.删除链表的倒数第N个节点

链接

快慢指针 + 虚拟头结点

# 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]:
        dummy = ListNode(next = head)

        fast = dummy
        slow = dummy

        for i in range(n):
            fast = fast.next 

        while fast.next:
            fast = fast.next
            slow = slow.next

        slow.next = slow.next.next

        return dummy.next

面试题 02.07. 链表相交

链接

等比例法

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution:
    def getIntersectionNode(self, headA: ListNode, headB: ListNode) -> ListNode:
        A, B = headA, headB

        while A != B:
            A = A.next if A else headB
            B = B.next if B else headA
        
        return B

142.环形链表II

链接

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution:
    def detectCycle(self, head: Optional[ListNode]) -> Optional[ListNode]:
        fast, slow = head, head

        while fast and fast.next:
            slow = slow.next
            fast = fast.next.next

            if slow == fast:
                slow = head

                while slow != fast:
                    slow = slow.next
                    fast = fast.next
                
                return slow
        
        return None