Remove Linked List Elements

前后夹心指针，cur指针是要删除的对象则pre指针指向cur.next然后保持原地不动，不是则按常规往前跳

class Solution(object):
    def removeElements(self, head, val):
        """
        :type head: ListNode
        :type val: int
        :rtype: ListNode
        """
        dummy = ListNode('a')
        dummy.next = head
        pre, cur = dummy, head

        while cur:
            if cur.val == val:
                pre.next = cur.next
                cur = cur.next
                continue
            pre = cur
            cur = cur.next

        return dummy.next