leetcode206

Reverse a singly linked list.

Example:

Input: 1->2->3->4->5->NULL
Output: 5->4->3->2->1->NULL
Follow up:

A linked list can be reversed either iteratively or recursively. Could you implement both?

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution:
    def reverseList(self, head: ListNode) -> ListNode:
        p = None
        
        while head:
            q = head
            head = q.next
            q.next = p
            p = q
            
        head = p
        return head

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution:
    def reverseList(self, head: ListNode) -> ListNode:
        return self.helper(head, None)
    
    def helper(self, head, node):
        if not head:
            return node
        temp = head.next
        head.next = node
        
        return self.helper(temp, head)