R3-双指针（快慢指针）

新建两个链表

一个链表记录<x的值

一个链表记录>=x的值

拼接即可

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def partition(self, head: Optional[ListNode], x: int) -> Optional[ListNode]:
        #新建链表以及指针表示
        low_dummy,high_dummy=ListNode(0),ListNode(0)
        low,high=low_dummy,high_dummy
        while head:
            if head.val<x:
                low.next=head
                low=low.next
            else:
                high.next=head
                high=high.next
            head=head.next
        #拼接
        low.next=high_dummy.next
        high.next=None
        return low_dummy.next

        

ps:

        #新建链表以及指针表示
        low_dummy,high_dummy=ListNode(0),ListNode(0)
        low,high=low_dummy,high_dummy