Leetcode刷题记录——143. 重排链表

Leetcode刷题记录——143. 重排链表_第1张图片

快慢指针找中点

反转后半段 注意将原后半段的头结点的next清空

将反转后的后半段的节点 插入到前半段的两两节点间

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def reorderList(self, head: ListNode) -> None:
        """
        Do not return anything, modify head in-place instead.
        """
        if head == None or head.next == None or head.next.next == None:
            return head
        
        slow = head
        fast = head
        while fast.next != None and fast.next.next != None:
            fast = fast.next.next
            slow = slow.next
        back_head = slow.next
        ##print(back_head)
        xx,reverse_head = self.reverse(back_head)
        
        xx.next = None
        slow.next = None
        cur1 = head
        last_next = reverse_head
        #print(reverse_head)
        while last_next != None:
            nextnext = cur1.next
            cur1.next = last_next
            last_next = last_next.next
            cur1.next.next = nextnext
            cur1 = cur1.next.next
        return head
    def reverse(self,root):
        if root == None:
            return None,None
        if root.next == None:
            return root,root
        elif root.next != None:
            new_head,reverse_head = self.reverse(root.next)
            new_head.next = root
            return root,reverse_head

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