reorderList.py

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def reorderList(self, head: Optional[ListNode]) -> None:
        """
        Do not return anything, modify head in-place instead.
        """
        slow = head
        fast = head
        while(fast and fast.next):
            slow = slow.next
            fast = fast.next.next

        prev = None
        curr = slow
        fwd = slow
        while(curr is not None):
            fwd = curr.next
            curr.next = prev
            prev = curr
            curr = fwd
        
        last = prev
        curr = head
        while(last.next is not None):
            fwd = curr.next
            curr.next = last

            lastFwd = last.next
            last.next = fwd

            curr = fwd
            last = lastFwd