def middle_node(self, head):
# write your code here
if not head:
return None
dummy = ListNode(-1)
dummy.next = head
slow, fast = dummy, dummy
while fast and fast.next:
slow = slow.next
fast = fast.next.next
return slow
def reorderList(self, head):
if head is None:
return head
#find mid
slow = head
fast = head
while fast.next and fast.next.next:
slow = slow.next
fast = fast.next.next
mid = slow
#cut in the mid
left = head
right = mid.next
if right is None:
return head
mid.next = None
#reverse right half
cursor = right.next
right.next = None
while cursor:
next = cursor.next
cursor.next = right
right = cursor
cursor = next
#merge left and right
dummy = ListNode(0)
while left or right:
if left is not None:
dummy.next = left
left = left.next
dummy = dummy.next
if right is not None:
dummy.next = right
right = right.next
dummy = dummy.next
return head