【leetcode】203. 移除链表元素（python）

写法一：不新建头结点

# Definition for singly-linked list.
# class ListNode(object):
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution(object):
    def removeElements(self, head, val):
        """
        :type head: ListNode
        :type val: int
        :rtype: ListNode
        """
        # 写法一：不新new一个头结点的写法
        if not head:   
            return head
        while head.val == val:
            if not head.next:
                return 
            head = head.next
        pre = head
        p = head.next
        while p:
            if p.val == val:
                pre.next = p.next
                p = p.next
            else:
                pre = p
                p = p.next
        return head

写法二：新建一个头结点，统一首元结点和链表中间结点的 *** 作

# Definition for singly-linked list.
# class ListNode(object):
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution(object):
    def removeElements(self, head, val):
        """
        :type head: ListNode
        :type val: int
        :rtype: ListNode
        """
        # 写法二：new 一个头结点，让头结点的指针指向首元结点，统一 *** 作（让首元结点和链表中间的结点的 *** 作一致）
        dummyNode = ListNode(0, head)
        pre = dummyNode
        p = head
        while p:
            if p.val == val:
                pre.next = p.next
                p = p.next
            else:
                pre = p
                p = p.next
        return dummyNode.next