添加虚拟节点作为头节点可以方便处理边界条件，比如少写几个if；
cur作为指针在链表上滑动
返回的是头节点
# Definition for singly-linked list.
# class ListNode:
# def __init__(self, val=0, next=None):
# self.val = val
# self.next = next
class Solution:
def removeElements(self, head: ListNode, val: int) -> ListNode:
dummy_head = ListNode(next=head) #添加一个虚拟节点
cur = dummy_head
while(cur.next!=None):
if(cur.next.val == val):
cur.next = cur.next.next #删除cur.next节点
else:
cur = cur.next
return dummy_head.next
注意改节点时的操作顺序
一定要知道操作节点的前一个节点的指针，因为单链表你一旦到了下一个节点就不知道（找不到）上一个节点了
#双指针
# Definition for singly-linked list.
# class ListNode:
# def __init__(self, val=0, next=None):
# self.val = val
# self.next = next
class Solution:
def reverseList(self, head: ListNode) -> ListNode:
cur = head
pre = None
while(cur!=None):
temp = cur.next # 保存一下 cur的下一个节点，因为接下来要改变cur->next
cur.next = pre #反转
#更新pre、cur指针
pre = cur
cur = temp
return pre
