def mergeTwoLists(self, l1: ListNode, l2: ListNode) -> ListNode:
if l1 is None:
return l2
elif l2 is None:
return l1
elif l1.val < l2.val:
l1.next = self.mergeTwoLists(l1.next, l2)
return l1
else:
l2.next = self.mergeTwoLists(l1, l2.next)
return l2
def reverseList(self, head: ListNode) -> ListNode:
if head is None or head.next is None:
return head
p = self.reverseList(head.next)
head.next.next = head
head.next = None
return p
def reverseList(self, head: ListNode) -> ListNode:
if head is None:
return None
prev = cur = ListNode(head.val)
while head.next:
prev = ListNode(head.next.val)
prev.next = cur
cur = prev
head = head.next
return prev
def removeNthFromEnd(self, head: ListNode, n: int) -> ListNode:
dummy = ListNode()
dummy.next = head
# use two pointers
first = dummy
second = dummy
for _ in range(n + 1):
first = first.next
while first:
first = first.next
second = second.next
second.next = second.next.next
return dummy.next
def removeNthFromEnd(self, head: ListNode, n: int) -> ListNode:
dummy = ListNode()
dummy.next = head
# count size
size = 0
node = head
while node:
size += 1
node = node.next
# find target's prev node and unlink target node
prev_node = dummy
for _ in range(size - n):
prev_node = prev_node.next
prev_node.next = prev_node.next.next
return dummy.next