[LeetCode] 206. Reverse Linked List

 Problem : https://leetcode.com/problems/reverse-linked-list/

The recursive solution:

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def reverseList(self, head: ListNode) -> ListNode:
        if not head or not head.next:
            # do not reverse empty list or single element list
            return head
        
        # reverse list starts with the tail element
        tail = head.next
        new_head = self.reverseList(tail)
        
        # append the previous head to the tail
        tail.next = head
        head.next = None
        
        # return the new head
        return new_head

The iterative solution:

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def reverseList(self, head: Optional[ListNode]) -> Optional[ListNode]:
        if not head: return None
        
        pre = head
        cur = head.next
        
        while cur:
            t = cur.next
            
            cur.next = pre
            pre = cur
            cur = t
        
        head.next = None
        return pre

Edited on 09/07/2021. Add the iterative solution.