206. Reverse Linked List

class Solution:
#     # iterative
#     def reverseList(self, head: ListNode) -> ListNode:
#         prev = None
#         curr = head
        
#         while curr:
#             # save next node
#             next_node = curr.next
#             # swap with previous node
#             curr.next = prev
#             # shift current node back
#             prev = curr
#             curr = next_node

#         return prev
    
    # recursive
    def reverseList(self, head: ListNode) -> ListNode:
        if (head == None) or (head.next == None):
            return head
        else:
            next_node = self.reverseList(head.next)
            # swap with next node's pointer
            head.next.next = head
            # erase head
            head.next = None
            return next_node