def partition(self, head, x):
    h1 = l1 = ListNode(0)
    h2 = l2 = ListNode(0)
    while head:
        if head.val < x:
            l1.next = head
            l1 = l1.next
        else:
            l2.next = head
            l2 = l2.next
        head = head.next
    l2.next = None
    l1.next = h2.next
    return h1.next