0. Structure of a Singly Linked List

class ListNode:
    def __init__(self, val=0, next=None):
        self.val = val
        self.next = next

// 1 -> 2 -> 3 -> 4 -> 5 -> NULL

1. Move pointers

def reverseList(head: ListNode) -> ListNode:
    prev = None
    curr = head

    while curr:
        temp = curr.next    # Save next node
        curr.next = prev    # Reverse the pointer
        prev = curr         # Move prev forward
        curr = temp         # Move curr forward

    return prev  # New head of the reversed list

0: 1 -> None 2 -> 3 -> 4 -> 5 -> NULL

1: 2 -> 1 -> None 3 -> 4 -> 5 -> NULL

2: 3 -> 2 -> 1 -> None 4 -> 5 -> NULL

3: 4 -> 3 -> 2 -> 1 -> None 5 -> NULL

4: 5 -> 4 -> 3 -> 2 -> 1 -> None

2. Head Insertion

def reverseList(head: ListNode) -> ListNode:
    prev = ListNode(0)
    prev.next = head
    curr = prev.next

    while curr.next:
        temp = curr.next
        curr.next = temp.next
        temp.next = prev.next
        prev.next = temp

    return prev.next

0: 1 -> 2 -> 3 -> 4 -> 5 -> NULL

1: 2 -> 1 -> 3 -> 4 -> 5 -> NULL

2: 3 -> 2 -> 1 -> 4 -> 5 -> NULL

3: 4 -> 3 -> 2 -> 1 -> 5 -> NULL

4: 5 -> 4 -> 3 -> 2 -> 1 -> NULL