题目要求：
Remove all elements from a linked list of integers that have value val.
Example：
Given:1 --> 2 --> 6 --> 3 --> 4 --> 5 --> 6, val= 6
Return: 1 --> 2 --> 3 --> 4 --> 5
解题思路：
- 需要临时头节点
- 遍历链表：while....
# Time: O(n)
# Space: O(1)
# Definition for singly-linked list.
# class ListNode:
# def __init__(self, x):
# self.val = x
# self.next = None
class Solution(object):
def removeElements(self, head, val):
"""
:type head: ListNode
:type val: int
:rtype: ListNode
"""
dummy = ListNode(float("-inf"))
dummy.next = head
prev, curr = dummy, dummy.next
while curr:
if curr.val == val:
prev.next = curr.next
else:
prev = curr
curr = curr.next
return dummy.next
