47移除链表元素

删除链表中等于给定值 val 的所有节点。

示例:
输入: 1->2->6->3->4->5->6, val = 6
输出: 1->2->3->4->5
迭代

class Solution:
    def removeElements(self, head: ListNode, val: int) -> ListNode:
        dummy = ListNode(0)
        dummy.next = head
        prev = dummy
        last = prev.next
        while last :
            if last.val == val:
                prev.next = last.next
                last = prev.next
            else:
                prev = prev.next
                last = prev.next
        return dummy.next

递归

class Solution:
    def removeElements(self, head: ListNode, val: int) -> ListNode:
        if not head: return 
        head.next = self.removeElements(head.next, val)
        return head.next if head.val == val else head

添加辅助头指针

class Solution:
    def removeElements(self, head: ListNode, val: int) -> ListNode:
        newH = ListNode(0)
        newH.next = head
        pre, p = newH, head
        while p:
            if p.val == val:
                pre.next = p = p.next
            else:
                pre, p = p, p.next
        return newH.next