203. 移除链表元素

class Solution(object):
    def removeElements(self, head, val):
        """
        :type head: ListNode
        :type val: int
        :rtype: ListNode
        """
        prehead = ListNode(-1)
        prehead.next = head
        pre = prehead
        while head:
            if head.val == val:
                pre.next = head.next
            else:
                pre = head
            head = head.next
        return prehead.next