给定一个链表和一个特定值 x，对链表进行分隔，使得所有小于 x 的节点都在大于或等于 x 的节点之前。
你应当保留两个分区中每个节点的初始相对位置。

示例:
输入: head = 1->4->3->2->5->2, x = 3
输出: 1->2->2->4->3->5

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

class Solution:
    def partition(self, head: ListNode, x: int) -> ListNode:
        if head is None or head.next is None:
            return head 
        left_head = ListNode(None)
        right_head = ListNode(None)
        cur_left = left_head
        cur_right = right_head
        cur = head 
        while cur:
            if cur.val < x:
                cur_left.next = cur 
                cur_left = cur_left.next
            else:
                cur_right.next = cur
                cur_right = cur_right.next
            cur = cur.next 
        cur_left.next = right_head.next 
        cur_right.next = None 
        return left_head.next