24.两两交换链表中的节点
难点：1. 理解交换链表后是如何连接的 2. 循环终止条件
可以优化卡哥的算法，将2.3的交换顺序调换，可以省去temp1变量指针。
# 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_head = ListNode(next=head)
cur = dummy_head
while cur.next and cur.next.next:
temp = cur.next
cur.next = cur.next.next
temp.next = cur.next.next
cur.next.next = temp
cur = cur.next.next
return dummy_head.next
19.删除链表的倒数第N个节点
重点：双指针，fast比slow多走N+1步，因为要删除倒数第N个节点需要知道倒数第N+1个节点。
# 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_head = ListNode(0,head)
slow = fast = dummy_head
for i in range(n+1):
fast = fast.next
while fast:
slow = slow.next
fast = fast.next
slow.next = slow.next.next
return dummy_head.next
02.07.链表相交
重点：链表元素末尾对齐
# 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:
lenA = self.getLength(headA)
lenB = self.getLength(headB)
if lenA > lenB:
headA = self.moveForward(headA, lenA-lenB)
else:
headB = self.moveForward(headB, lenB-lenA)
while headA and headB:
if headA == headB:
return headA
headA = headA.next
headB = headB.next
return None
def getLength(self, head: ListNode) -> int:
length = 0
while head:
head = head.next
length += 1
return length
def moveForward(self, head: ListNode, steps:int) -> int:
while steps:
head = head.next
steps -= 1
return head
142.环形链表II
重点：找到环入口
# 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]:
slow = head
fast = head
while fast and fast.next:
fast = fast.next.next
slow = slow.next
# 判断是否有环
if fast == slow:
index1 = fast
index2 = head
while index1 != index2:
index1 = index1.next
index2 = index2.next
return index1
return None
