0206 Reverse Linked List

Question

Python

My Solution 1: Two pointer

# 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]:
        prev, curr = None, head

        while curr:
            nxt = curr.next # save the next node
            curr.next = prev
            prev = curr
            curr = nxt
        
        return prev

My Solution 2: Recursive

# 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
        
        newHead = head
        if head.next:
            newHead = self.reverseList(head.next)
            head.next.next = head
        head.next = None

        return newHead