Reverse Linked List

206. Reverse Linked List

Difficulty: Easy

Related Topics: Linked List, Recursion

Given the head of a singly linked list, reverse the list, and return the reversed list.

Example 1:

Input: head = [1,2,3,4,5] Output: [5,4,3,2,1]

Example 2:

Input: head = [1,2] Output: [2,1]

Example 3:

Input: head = [] Output: []

Code:

# 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]:
        # iterative solution
        prev, curr = None, head
        while curr:
            nxt = curr.next
            curr.next = prev
            prev = curr
            curr = nxt
        return prev

        # recursive solution
        if not head:
            return None
        
        newHead = head
        if head.next:
            newHead = self.reverseList(head.next)
            head.next.next = head
        head.next = None

        return newHead